Projection optical system

• US 7,957,069 B2
• Filed: 12/30/2005
• Issued: 06/07/2011
• Est. Priority Date: 12/30/2004
• Status: Active Grant

• Current Assignee: CARL Zeiss SMT GmbH (Carl-Zeiss-Stiftung)
• Original Assignee: CARL Zeiss SMT GmbH (Carl-Zeiss-Stiftung)
• Inventors: Ulrich, Wilhelm, Dodoc, Aurelian
• Primary Examiner(s): Harrington; Alicia M
• Application Number:
  US11/793,678
• Publication Number:
  US 20070268594A1
• Time in Patent Office: 1,985 Days
• Field of Search: 359/649, 359/650, 359/686, 359/691, 359/715, 359/717, 359/781, 359/753, 359/793
• US Class Current: 359/650
• CPC Class Codes: G02B 13/143 for use with ultraviolet radiation
• CPC Class Codes: G03F 7/70225 Optical aspects of catadioptric systems, i.e. comprising reflective and refractive elements
• CPC Class Codes: G03F 7/70241 Optical aspects of refractive lens systems, i.e. comprising only refractive elements

Abstract

A projection optical system comprises a plurality of lenses disposed along an optical axis of the projection optical system; wherein the plurality of lenses is dividable into four non-overlapping groups of lenses, such that a total refractive power of each group of lenses is one of a negative refractive power and a positive refractive power; and wherein a refractive power of each lens of the fourth group of lenses is equal to or greater than 0. A lens of the third group of lenses which is disposed directly adjacent to a lens of the fourth group of lenses may have a concave surface facing towards the second object.

Claims

1. A projection optical system for imaging a first object into a region of a second object, the projection optical system comprising:

a plurality of lenses disposed along an optical axis of the projection optical system;

wherein the plurality of lenses is dividable into a plurality of non-overlapping groups of lenses, such that a total refractive power of each group of lenses is one of a negative refractive power and a positive refractive power;

wherein a first group of lenses has a negative refractive power;

wherein a lens closest to the first object of the plurality of lenses is included in the first group of lenses;

wherein a refractive power of each lens of the first group of lenses is a negative refractive power;

wherein a second group disposed directly adjacent to the first group has a positive refractive power;

wherein a third group disposed directly adjacent to the second group has a negative refractive power;

wherein a fourth group disposed directly adjacent to the third group has a positive refractive power;

wherein the fourth group of lenses is that group of lenses among all groups of lenses that is disposed closest to the second object;

wherein a refractive power of each lens of the fourth group of lenses is equal to or greater than 0,

wherein a lens of the third group of lenses which is disposed directly adjacent to a lens of the fourth group of lenses has a concave surface facing towards the second object, and

wherein the fourth group of lenses further comprises an aperture stop.

2. The projection optical system according to claim 1, wherein the refractive power of each lens of the fourth group of lenses is greater than 0.

3. The projection optical system according to claim 1, having a numerical aperture on a side of the second object of 0.7 or greater.

4. A projection optical system for imaging a first object into a region of a second object, the projection optical system comprising:

a plurality of lenses disposed along an optical axis of the projection optical system;

wherein the plurality of lenses is dividable into a plurality of non-overlapping groups of lenses, such that a total refractive power of each group of lenses is one of a negative refractive power and a positive refractive power;

wherein a first group of lenses has a negative refractive power;

wherein a lens closest to the first object of the plurality of lenses is included in the first group of lenses;

wherein a refractive power of each lens of the first group of lenses is a negative refractive power;

wherein a second group disposed directly adjacent to the first group has a positive refractive power;

wherein a third group disposed directly adjacent to the second group has a negative refractive power;

wherein a fourth group disposed directly adjacent to the third group has a positive refractive power;

wherein the fourth group of lenses is that group of lenses among all groups of lenses that is disposed closest to the second object;

wherein a refractive power of each lens of the fourth group of lenses is equal to or greater than 0,

wherein a diameter of a bundle of imaging rays imaging a first location on the optical axis disposed in a region of the first object into a second location on the optical axis disposed in a region of the second object has a minimum cross section at a third location on the optical axis disposed within the third lens group, and

wherein only one pair of lenses disposed between the third location and the second location have opposite refractive powers such that a refractive power of a first lens of the pair of lenses and a refractive power of a second lens of the pair of lenses have opposite signs.

5. The projection optical system according to claim 4, wherein an aperture stop is disposed at a fourth location on the optical axis, and wherein the one pair of lenses is disposed between the third location and the fourth location.

6. The projection optical system according to claim 4 wherein a lens of the third group of lenses which is disposed directly adjacent to a lens of the fourth group of lenses has a concave surface facing towards the second object.

7. The projection optical system according to claim 4, wherein a largest effective diameter among effective diameters of the lenses having a refractive power of equal to or greater than 0 is by a factor of between 1.1 and 2.5 greater than an effective diameter of any lens having a negative refractive power.

8. The projection optical system according to claim 4 wherein an effective diameter of any lens having a negative refractive power is less than L/5, wherein L is a distance between the first object and the second object.

9. The projection optical system according to claim 4, wherein each lens of the first and third groups of lenses has a smaller diameter than any of the lenses of the second group of lenses.

10. The projection optical system according to claim 4, wherein a refractive power of each lens of the second group of lenses is greater than or equal to zero.

11. The projection optical system according to claim 4, wherein a refractive power of each lens of the third group of lenses is a negative refractive power.

12. The projection optical system according to claim 11, wherein the third group of lenses comprises at least two lenses.

13. The projection optical system according to claim 4, wherein at least one lens of the fourth group of lenses has at least one aspherical surface, wherein an axial distance between the aspherical surface and a best fitting sphere of the aspherical surface is more than about 1.0 mm.

14. The projection optical system according to claim 13, wherein the fourth group of lenses further comprises an aperture stop and wherein the at least one aspherical lens is disposed in between the aperture stop and the second object.

15. The projection optical system according to claim 4, wherein at least two lenses of the lenses of the third and fourth groups of lenses have at least one aspherical surface, wherein an axial distance between the respective aspherical surface and a best fitting sphere thereof is more than about 1.0 mm.

16. The projection optical system according to claim 4, wherein at least three lenses of the lenses of the third and fourth groups of lenses have at least one aspherical surface, wherein an axial distance between the respective aspherical surface and a best fitting sphere thereof is more than about 1.0 mm.

17. The projection optical system according to claim 4, having a region extending from the first object along the optical axis towards the second object, in which region the following condition is fulfilled:

|h_au/h_fc|<

1.2, wherein h_au is a distance between the optical axis and a field ray that originates from a point of the first object having a maximum distance from the optical axis, wherein the field ray originates from that point in a direction parallel to the optical axis, and h_fc is a distance between the optical axis and an angular ray originating from an axial point of the first object, the angular ray forming a largest possible angle with the optical axis such that the ray contributes to imaging of the first object into the region of the second object, wherein h_au and h_fc are measured at the same location on the optical axis; and

wherein said region extends over a distance of at least one third of an overall distance between the first and the second object.

18. The projection optical system according to claim 17, wherein at least two aspherical surfaces are disposed in said region.

19. The projection optical system according to claim 4, wherein a distance on the optical axis between any two directly adjacent lenses of the second group of lenses is smaller than L/100, wherein L is a distance between the first object and the second object.

20. The projection optical system according to claim 4, wherein at least one distance on the optical axis between two directly adjacent lenses of the second group of lenses is greater than L/50, wherein L is a distance between the first object and the second object.

21. The projection optical system according to claim 4, wherein a ratio of a difference between a total length of the fourth group of lenses and a sum of thicknesses of all lenses of the fourth group of lenses over the total length of the fourth group of lenses is greater than 0.3.

22. The projection optical system according to claim 4, wherein the first group of lenses comprises at least two lenses.

23. The projection optical system according to claim 4, wherein the fourth group of lenses comprises at least five lenses.

24. The projection optical system according to claim 4, wherein the fourth group of lenses comprises an aperture stop and wherein at least three lenses are disposed in between the aperture stop and the second object.

25. The projection optical system according to claim 4, wherein the fourth group of lenses comprises an aperture stop and wherein at least two lenses are disposed in between the aperture stop and the second object.

26. The projection optical system according to claim 4, wherein at least two lenses of the fourth group of lenses are made of a crystalline material having an intrinsic birefringence.

27. The projection optical system according to claim 26, wherein crystal orientations of the at least two lenses of the crystalline material are oriented relative to each other such that at least a portion of a birefringence of a first lens of the two lenses of the crystalline material is compensated by a second lens of the two lenses of the crystalline material.

28. The projection optical system according to claim 26, wherein the crystalline material comprises CaF₂.

29. The projection optical system according to claim 4, wherein at least one lens of the fourth group of lenses is an aspherical lens having a surface of an aspherical shape such that the surface has, when seen in a radial section, at least one concave portion and at least one convex portion.

30. The projection optical system according to claim 29, wherein the surface of the at least one aspherical lens has, when seen in a radial direction, two concave portions and one convex portion between the two concave portions.

31. The projection optical system according to claim 4, wherein, on the optical axis, at least one distance between two directly adjacent lenses of the fourth group of lenses is greater than L/50, wherein L is a distance between the first object and the second object.

32. The projection optical system according to claim 4, wherein the projection optical system is configured such that the second object is disposed at a working distance from 2 mm to 10 mm from a lens of the projection optical system disposed closest to the second object when in focus.

33. The projection optical system according to claim 4, having a numerical aperture on a side of the second object of 0.9 or greater.

34. A projection optical system for imaging a first object into a region of a second object, the projection optical system comprising:

a plurality of lenses disposed along an optical axis of the projection optical system;

wherein the plurality of lenses is dividable into two non-overlapping groups of lenses,

wherein a first main group of lenses of the two non-overlapping groups of lenses comprises a lens closest to the first object;

wherein a second main group of lenses of the two non-overlapping groups of lenses comprises a lens closest to the second object;

wherein the plurality of lenses is divided into the first and second main groups of lenses such that a refractive power of the first main group of lenses has a maximum negative value;

wherein lens surfaces of the first and second main groups of lenses include a plurality of aspherical lens surfaces, and

wherein a first aspherical lens surface of the plurality of aspherical lens surfaces is configured such that an axial distance between the aspherical lens surface and a best fitting sphere thereof is more than about 1.0 mm.

35. The projection optical system according to claim 34, wherein the axial distance between the aspherical lens surface and the best fitting sphere thereof is more than about 1.5 mm.

36. The projection optical system according to claim 34, wherein the first aspherical lens surface has a central portion traversed by an optical axis of the projection optical system and a ring portion disposed outside of the central portion, and wherein a local refractive power of the lens experienced by a light beam traversing the central portion and a local refractive power of the lens experienced by a light beam traversing the ring portion have opposite signs.

37. The projection optical system according to claim 34, wherein the second main group of lenses further comprises an aperture stop.

38. The projection optical system according to claim 34, wherein the first main group of lenses is dividable into a plurality of non-overlapping sub-groups of lenses such that a total refractive power of each sub-group of lenses is one of a negative refractive power and a positive refractive power.

39. The projection optical system according to claim 34, wherein the first main group of lenses is dividable into three non-overlapping sub-groups of lenses, wherein a first sub-group of lenses of the three non-overlapping sub-groups of lenses has a negative refractive power and is disposed closest to the first object, wherein a second sub-group of lenses of the three non-overlapping subgroups of lenses has a positive refractive power, and wherein a third sub-group of lenses of the three non-overlapping sub-groups of lenses has a negative refractive power.

40. The projection optical system according to claim 39, wherein a refractive power of each lens of the first sub-group of lenses is a negative refractive power.

41. The projection optical system according to claim 39, wherein a refractive power of each lens of the third sub-group of lenses is a negative refractive power.

42. The projection optical system according to claim 41, wherein the third sub-group of lenses comprises at least two lenses.

43. The projection optical system according to claim 34, having a region extending from the first object along the optical axis towards the second object, in which region the following condition is fulfilled:

|h_au/h_fc|<

1.2, wherein h_au is a distance between the optical axis and a field ray that originates from a point of the first object having a maximum distance from the optical axis, wherein the field ray originates from that point in a direction parallel to the optical axis, and h_fc is a distance between the optical axis and an angular ray originating from an axial point of the first object, the angular ray forming a largest possible angle with the optical axis such that the ray contributes to imaging of the first object into the region of the second object, wherein h_au and h_fc are measured at the same location on the optical axis; and

wherein said region extends over a distance of at least one third of an overall distance between the first and the second object.

44. The projection optical system according to claim 43, wherein at least two aspherical surfaces are disposed in said region.

45. The projection optical system according to claim 34, wherein the second main group of lenses comprises at least five lenses.

46. The projection optical system according to claim 34, wherein the second main group of lenses comprises an aperture stop and wherein at least three lenses are disposed in between the aperture stop and the second object.

47. The projection optical system according to claim 34, wherein the second main group of lenses comprises an aperture stop and wherein at least two lenses are disposed in between the aperture stop and the second object.

48. The projection optical system according to claim 34, wherein at least two lenses of the second main group of lenses are made of a crystalline material having an intrinsic birefringence.

49. The projection optical system according to claim 48, wherein crystal orientations of the at least two lenses of the crystalline material are oriented relative to each other such that at least a portion of a birefringence of a first lens of the two lenses of the crystalline material is compensated by a second lens of the two lenses of the crystalline material.

50. The projection optical system according to claim 48, wherein the crystalline material comprises CaF₂.

51. The projection optical system according to claim 34, wherein, on the optical axis, at least one distance between two directly adjacent lenses of the second main group of lenses is greater than L/50, wherein L is a distance between the first object and the second object.

52. The projection optical system according to claim 34, wherein the projection optical system is configured such that the second object is disposed at a working distance from 2 mm to 10 mm from a lens of the projection optical system disposed closest to the second object when in focus.

53. The projection optical system according to claim 34, having a numerical aperture on a side of the second object of 0.7 or greater.

54. The projection optical system according to claim 34, having a numerical aperture on a side of the second object of 0.9 or greater.

55. The projection optical system according to claim 34, wherein the projection optical system is a catadioptric projection optical system.

56. A projection optical system for imaging a first object into a region of a second object, the projection optical system comprising:

a plurality of lenses disposed along an optical axis of the projection optical system;

wherein the plurality of lenses is dividable into two non-overlapping groups of lenses,

wherein a first main group of lenses of the two non-overlapping groups of lenses comprises a lens closest to the first object;

wherein a second main group of lenses of the two non-overlapping groups of lenses comprises a lens closest to the second object;

wherein the plurality of lenses is divided into the first and second main groups of lenses such that a refractive power of the first main group of lenses has a maximum negative value; and

wherein lens surfaces of the first and second main groups of lenses include a plurality of aspherical lens surfaces, and

wherein a first aspherical lens surface of the plurality of aspherical lens surfaces has a central portion traversed by an optical axis of the projection optical system and a ring portion disposed outside of the central portion, and

wherein a local refractive power of the lens experienced by a light beam traversing the central portion and a local refractive power of the lens experienced by a light beam traversing the ring portion have opposite signs.

57. The projection optical system according to claim 56, wherein the first aspherical lens surface is configured such that an axial distance between the aspherical lens surface and a best fitting sphere thereof is more than about 1.0 mm.

58. The projection optical system according to claim 56, wherein the first aspherical lens surface is configured such that an axial distance between the aspherical lens surface and a best fitting sphere thereof is more than about 1.5 mm.

59. A projection optical system configured to image a first object into a region of a second object, the projection optical system comprising:

a plurality of lenses disposed along an optical axis of the projection optical system;

wherein;

the plurality of lenses is dividable into a plurality of non-overlapping groups of lenses, such that a total refractive power of each group of lenses is one of a negative refractive power and a positive refractive power;

the groups are selected such that a sum of absolute values of the total refractive powers of the groups is a maximum value;

a first group of lenses has a negative refractive power;

a lens closest to the first object of the plurality of lenses is included in the first group of lenses;

a refractive power of each lens of the first group of lenses is a negative refractive power;

a second group disposed directly adjacent to the first group has a positive refractive power;

a third group disposed directly adjacent to the second group has a negative refractive power;

a fourth group disposed directly adjacent to the third group has a positive refractive power;

the fourth group of lenses is that group of lenses among all groups of lenses that is disposed closest to the second object;

a refractive power of each lens of the fourth group of lenses is equal to or greater than 0, and

a lens of the third group of lenses which is disposed directly adjacent to a lens of the fourth group of lenses has a concave surface facing towards the second object.

60. The projection optical system according to claim 59, wherein a largest effective diameter among effective diameters of the lenses having a refractive power of equal to or greater than 0 is by a factor of between 1.1 and 2.5 greater than an effective diameter of any lens having a negative refractive power.

61. The projection optical system according to claim 59, wherein an effective diameter of any lens having a negative refractive power is less than L/5, wherein L is a distance between the first object and the second object.

62. The projection optical system according to claim 59, wherein a refractive power of each lens of the third group of lenses is a negative refractive power.

63. The projection optical system according to claim 59, wherein at least one lens of the fourth group of lenses has at least one aspherical surface, and an axial distance between the aspherical surface and a best fitting sphere of the aspherical surface is more than about 1.0 mm.

64. The projection optical system according to claim 59, wherein the projection optical system has a region extending from the first object along the optical axis towards the second object, in which region the following condition is fulfilled:

|h_au/h_fc|<

1.2,

wherein;

h_au is a distance between the optical axis and a field ray that originates from a point of the first object having a maximum distance from the optical axis, wherein the field ray originates from that point in a direction parallel to the optical axis; and

h_fC is a distance between the optical axis and an angular ray originating from an axial point of the first object, the angular ray forming a largest possible angle with the optical axis such that the ray contributes to imaging of the first object into the region of the second object;

h_au and h_fC are measured at the same location on the optical axis; and

said region extends over a distance of at least one third of an overall distance between the first and the second object.

65. The projection optical system according to claim 59, wherein a ratio of a difference between a total length of the fourth group of lenses and a sum of thicknesses of all lenses of the fourth group of lenses over the total length of the fourth group of lenses is greater than 0.3.

66. The projection optical system according to claim 59, wherein at least two lenses of the fourth group of lenses are made of a crystalline material having an intrinsic birefringence.

67. The projection optical system according to claim 59, wherein at least one lens of the fourth group of lenses is an aspherical lens having a surface of an aspherical shape such that the surface has, when seen in a radial section, at least one concave portion and at least one convex portion.

68. The projection optical system according to claim 59, wherein the projection optical system is configured such that the second object is disposed at a working distance from 2 mm to 10 mm from a lens of the projection optical system disposed closest to the second object when in focus.

69. The projection optical system according to claim 59, having a numerical aperture on a side of the second object of 0.9 or greater.

70. A projection optical system for imaging a first object into a region of a second object, the projection optical system comprising:

a plurality of lenses disposed along an optical axis of the projection optical system;

wherein the plurality of lenses is dividable into two non-overlapping groups of lenses,

wherein a first main group of lenses of the two non-overlapping groups of lenses comprises a lens closest to the first object;

wherein a second main group of lenses of the two non-overlapping groups of lenses comprises a lens closest to the second object;

wherein the plurality of lenses is divided into the first and second main groups of lenses such that a refractive power of the first main group of lenses has a maximum negative value;

wherein a refractive power of each lens of the second main group of lenses is equal to or greater than 0,

wherein the projection optical system has a region extending from the first object along the optical axis towards the second object, in which region the following condition is fulfilled;

|h_au/h_fc|<

1.2,

wherein h_au is a distance between the optical axis and a field ray that originates from a point of the first object having a maximum distance from the optical axis, wherein the field ray originates from that point in a direction parallel to the optical axis,

wherein h_fc is a distance between the optical axis and an angular ray originating from an axial point of the first object, the angular ray forming a largest possible angle with the optical axis such that the ray contributes to imaging of the first object into the region of the second object,

wherein h_au and h_fc are measured at the same location on the optical axis; and

wherein said region extends over a distance of at least one third of an overall distance between the first and the second object.

71. A projection optical system for imaging a first object into a region of a second object, the projection optical system comprising:

a plurality of lenses disposed along an optical axis of the projection optical system;

wherein the plurality of lenses is dividable into two non-overlapping groups of lenses,

wherein a first main group of lenses of the two non-overlapping groups of lenses comprises a lens closest to the first object;

wherein a second main group of lenses of the two non-overlapping groups of lenses

comprises a lens closest to the second object;

wherein the plurality of lenses is divided into the first and second main groups of lenses such that a refractive power of the first main group of lenses has a maximum negative value;

wherein a refractive power of each lens of the second main group of lenses is equal to or greater than 0, and

wherein at least two lenses of the second main group of lenses are made of a crystalline material having an intrinsic birefringence.

72. The projection optical system according to claim 71, wherein the refractive power of each lens of the second main group of lenses is greater than 0.

73. A projection optical system for imaging a first object into a region of a second object, the projection optical system comprising:

a plurality of lenses disposed along an optical axis of the projection optical system;

wherein the plurality of lenses is dividable into two non-overlapping groups of lenses,

wherein a first main group of lenses of the two non- overlapping groups of lenses comprises a lens closest to the first object;

wherein a second main group of lenses of the two non-overlapping groups of lenses comprises a lens closest to the second object;

wherein the plurality of lenses is divided into the first and second main groups of lenses such that a refractive power of the first main lens group has a maximum negative value;

wherein the plurality of lenses comprises one largest lens having a surface having a largest effective diameter out of all surface of lenses of the plurality of lenses, the one largest lens being contained in either the first or second main group of lenses,

wherein each of the plurality of lenses which is a meniscus lens and which has a surface having an effective diameter that is at least 50% of the largest effective diameter and which is comprised in the either first or second main group of lenses containing the largest lens fulfils the following requirement;

|r₁|≧

220 mm and |r₂|≧

220 mm, and

wherein r₁ is a radius of curvature of a first optical surface of the meniscus lens, and r₂ is a radius of curvature of a second optical surface of the meniscus lens.

74. The projection optical system according to claim 73, wherein each meniscus lens which has a surface having an effective diameter that is at least 70% of the largest effective diameter and which is comprised in the either first or second main group of lenses containing the largest lens fulfils the requirement.

75. The projection optical system according to claim 73, wherein each meniscus lens comprised in the either first or second main group of lenses further fulfils at least one of the following requirements:

|r₁|≧

300 mm and |r₂|≧

300 mm.

76. A projection optical system for imaging a first object into a region of a second object, the projection optical system comprising:

a plurality of lenses disposed along an optical axis of the projection optical system;

wherein the plurality of lenses is dividable into two non-overlapping groups of lenses,

wherein a first main group of lenses of the two non-overlapping groups of lenses comprises a lens closest to the first object;

wherein a second main group of lenses of the two non-overlapping groups of lenses comprises a lens closest to the second object;

wherein the plurality of lenses is divided into the first and second main groups of lenses such that a refractive power of the first main lens group has a maximum negative value;

wherein the plurality of lenses comprises one largest lens having a surface having a largest effective diameter out of all surface of lenses of the plurality of lenses, the one largest lens being contained in either the first or second main group of lenses,

wherein each of the plurality of lenses which is a meniscus lens and which has a surface having an effective diameter that is at least 50% of the largest effective diameter and which is comprised in the either first or second main group of lenses containing the largest lens fulfils the following requirement;

|r₁|≧

140 mm and |r₂|≧

140 mm,

wherein r₁ is a radius of curvature of a first optical surface of the meniscus lens, and r₂ is a radius of curvature of a second optical surface of the meniscus lens,wherein each meniscus lens comprised in the either first or second main group of lenses further fulfils the following requirement;

|r₁/r₂>

1.5; and

wherein r₁ indicates the radius of curvature of the concave surface of the meniscus lens.

77. A projection optical system for imaging a first object into a region of a second object, the projection optical system comprising:

a plurality of lenses disposed along an optical axis of the projection optical system;

wherein the plurality of lenses is dividable into two non-overlapping groups of lenses,

wherein a first main group of lenses of the two non-overlapping groups of lenses comprises a lens closest to the first object;

wherein a second main group of lenses of the two non-overlapping groups of lenses comprises a lens closest to the second object;

wherein the plurality of lenses is divided into the first and second main groups of lenses such that a refractive power of the first main lens group has a maximum negative value;

wherein the plurality of lenses comprises one largest lens having a surface having a largest effective diameter out of all surface of lenses of the plurality of lenses, the one largest lens being contained in either the first or second main group of lenses,

wherein each of the plurality of lenses which is a meniscus lens and which has a surface having an effective diameter that is at least 50% of the largest effective diameter and which is comprised in the either first or second main group of lenses containing the largest lens fulfils the following requirement;

|r₁|≧

140 mm and |r₂|≧

140 mm,

Wherein r₁ is a radius of curvature of a first optical surface of the meniscus lens, and r₂ is a radius of curvature of a second optical surface of the meniscus lens, and

wherein each meniscus lens comprised in the either first or second main group of lenses further has an effective diameter which is less than 90% of the largest effective diameter.