Structures and methods for reducing aberration in optical systems

• US 20030086156A1
• Filed: 06/20/2002
• Published: 05/08/2003
• Est. Priority Date: 10/30/2001
• Status: Active Grant

• Current Assignee: ASML Netherlands BV (ASML Holding NV)
• Original Assignee: ASML Netherlands BV (ASML Holding NV)
• Inventors: McGuire, James P. Jr.
• Granted Patent: US 7,453,641 B2
• Time in Patent Office: Days
• US Class Current: 359/352
• CPC Class Codes: G02B 1/08 made of polarising materials
• CPC Class Codes: G02B 27/28 for polarising used in stereoscopes G02B30/25
• CPC Class Codes: G03F 7/70075 Homogenization of illumination intensity in the mask plane by using an integrator, e.g. fly's eye lens, facet mirror or glass rod, by using a diffusing optical element or by beam deflection
• CPC Class Codes: G03F 7/70216 Mask projection systems
• CPC Class Codes: G03F 7/70966 Birefringence

Abstract

An optical system includes multiple cubic crystalline optical elements and one or more polarization rotators in which the crystal lattices of the cubic crystalline optical elements are oriented with respect to each other to reduce the effects of intrinsic birefringence and produce a system with reduced retardance. The optical system may be a refractive or catadioptric system having a high numerical aperture and using light with a wavelength at or below 248 nanometers. The net retardance of the system is less than the sum of the retardance contributions of the respective optical elements. In one embodiment, all cubic crystalline optical elements are oriented with identical three dimensional cubic crystalline lattice directions, a 90° polarization rotator divides the system into front and rear groups such that the net retardance of the front group is balanced by the net retardance of the rear group. The optical system may be used in a photolithography tool to pattern substrates such as semiconductor substrates and thereby produce semiconductor devices.

Claims

1. An optical system comprising:

at least one cubic crystalline optical element aligned along an optical axis, said at least one cubic crystalline optical element being birefringent and imparting retardance on a beam of light propagating through said optical system along said optical axis; and

polarization rotation optics inserted along said optical axis to rotate the polarization of said beam of light to reduce said retardance.

2. The optical system of claim 1, wherein said at least one cubic crystalline optical element comprises at least one powered optical element.

3. The optical element of claim 2, wherein said at least one cubic crystalline optical element is selected from the group consisting of refractive elements, catadioptric elements, and diffractive elements.

4. The optical system of claim 2, wherein said optical system is an imaging system.

5. The optical system of claim 1, wherein said at least one cubic crystalline optical element comprises at least one non-powered optical element.

6. The optical system of claim 1, wherein said at least one cubic crystalline optical element comprises at least one [111] cubic crystalline optical element aligned with its respective [111] lattice direction substantially parallel to said optical axis passing through said element.

7. The optical system of claim 1, wherein said at least one cubic crystalline optical element comprises at least one [100] cubic crystalline optical element aligned with its respective [100] lattice direction substantially parallel to said optical axis passing through said element.

8. The optical system of claim 1, wherein said at least one cubic crystalline optical element comprise at least one [110] cubic crystalline optical element aligned with its respective [110] lattice direction substantially parallel to said optical axis passing through said element.

9. The optical system of claim 1, wherein said at least one cubic crystalline optical element comprises material selected from the group consisting of calcium fluoride, barium fluoride, lithium fluoride, and strontium fluoride.

10. The optical system of claim 9, wherein said at least one cubic crystalline optical element comprises calcium fluoride.

11. The optical system of claim 1, comprising a plurality of said cubic crystalline optical elements having different crystal lattice orientations aligned along said optical axis.

12. The optical system of claim 11, wherein at least one of said cubic crystalline optical elements is rotated about the optical axis with respect to another of said cubic crystalline optical elements to reduce said retardance.

13. The optical system of claim 1, wherein said polarization rotation optics comprises a ±

90(2n+1) degree rotator that rotates an input polarization by ±

90(2n+1) degrees, where n is an integer.

14. The optical system of claim 1, wherein said polarization rotation optics comprises a circular retarder having orthogonal circular eigenpolarization states.

15. The optical system of claim 14, wherein said circular retarder comprises a 90°

circular retarder that imparts a +90(2n+1) degree phase shift between said orthogonal circular eigenpolarization states, where n is an integer.

16. The optical system of claim 1, where said polarization rotation optics comprises one or more waveplates.

17. The optical system of claim 16, wherein said waveplates are selected from the group consisting of retarders comprising uniaxial crystalline material and retarders comprising material having stress-induced birefringence resulting from externally applied stress.

18. The optical system of claim 16, where said waveplates are selected from the group consisting of quarter-wave plates and half-wave plates.

19. The optical system of claim 18, wherein said polarization rotation optics comprises two crossed quarter-wave plates having fast axes oriented 90 degrees with respect to each other about said optical axis, and a half-wave plate having a fast axis oriented at 45 degrees about said optical axis with respect to the axes of said quarter-wave plates.

20. The optical system of claim 16, wherein at least one of said one or more waveplates comprises cubic crystal.

21. The optical system of claim 16, wherein at least one of said one or more waveplates comprises a [100] cubic crystal aligned with its [100] lattice direction substantially parallel with said optical axis.

22. The optical system of claim 16, wherein at least one of said one or more waveplates comprises a [111] cubic crystal aligned with its [111] lattice direction substantially parallel with said optical axis.

23. The optical system of claim 16, wherein at least one of said one or more waveplates comprises calcium fluoride.

24. The optical system of claim 16, wherein at least one of said one or more waveplates comprises non-cubic crystalline material.

25. The optical system of claim 1, where said polarization rotation optics includes at least one powered optical element.

26. The optical system of claim 25, where said at least one powered optical element in said polarization rotation optics has at least one curved surface.

27. The optical system of claim 1, wherein said polarization rotation optics comprises a plurality of polarization rotators.

28. The optical system of claim 27, wherein said polarization rotation optics comprises a plurality of polarization rotators together rotate polarization by ±

90(2n+1) degrees, where n is an integer.

29. The optical system of claim 1, where said polarization rotation optics comprises a Faraday rotator.

30. The optical system of claim 1, comprising a plurality of said cubic crystalline optical elements, wherein said polarization rotation optics is disposed between cubic crystalline optical elements having substantially similar retardance.

31. The optical system of claim 1, further comprising at least one non-cubic crystalline optical element.

32. The optical system of claim 31, wherein said at least one non-cubic crystalline optical element comprises fused silica.

33. An optical apparatus having an output comprising:

a plurality of optical elements divided into first and second sections, said first and second sections having associated therewith polarization aberrations originating from variation in optical properties of said respective sections with polarization, said polarization aberrations affecting said output of said optical apparatus, said polarization aberrations associated with said first section being substantially similar to said polarization aberrations associated with said second section; and

polarization conversion optics disposed between said first and second optical sections, said polarization conversion optics configured to transform an input polarization into a orthogonal output polarization such that said polarization aberrations associated with said first section at least partially offset said polarization aberrations associated with said second section and thereby reduce said effects of polarization aberrations on said output of said optical system.

34. The optical apparatus of claim 33, wherein said plurality of optical elements comprise [111] cubic crystalline optical elements aligned with their respective [111] lattice directions substantially parallel with an optical axis passing therethrough.

35. The optical apparatus of claim 34, wherein said first and second sections comprise [111] cubic crystalline calcium fluoride having a [111] lattice direction substantially parallel with an optical axis passing therethrough.

36. The optical apparatus of claim 33, wherein said plurality of optical elements comprise [100] cubic crystalline optical elements aligned with their respective [100] lattice directions substantially parallel with an optical axis passing therethrough.

37. The optical apparatus of claim 33, wherein said plurality of optical elements comprise [110] cubic crystalline optical elements aligned with their respective [110] lattice directions substantially parallel with an optical axis passing therethrough.

38. The apparatus of claim 33, wherein one or more optical elements comprise calcium fluoride optical elements.

39. The optical apparatus of claim 33, wherein said polarization aberration includes diattenuation, and said polarization conversion optics are configured to reduce said diattenuation.

40. The optical apparatus of claim 33, wherein said polarization aberration includes retardance, and said polarization conversion optics are configured to reduce said retardance.

41. The optical apparatus of claim 33, wherein said first and second sections have substantially similar retardance.

42. The optical apparatus of claim 33, wherein said optical conversion optic comprise an optical rotator.

43. The optical apparatus of claim 33, further comprising a light source disposed with respect to the plurality of optical elements to propagate light therethrough.

44. The optical system of claim 33, wherein said light source comprises an excimer laser.

45. The optical system of claim 33, wherein said optical system is a non-imaging system.

46. An optical imaging system for producing an optical image, comprising:

one or more powered optical elements with polarization aberration that degrade said optical image; and

a polarization rotation system configured to reduce said contributions of said polarization aberration to said degradation of said optical image.

47. The optical imaging system of claim 46, wherein said polarization aberration includes diattenuation, and said polarization rotation system is configured to reduce said diattenuation.

48. The optical imaging system of claim 46, wherein said polarization aberration includes retardance, and said polarization rotation system is configured to reduce said retardance.

49. The optical imaging system of claim 46, wherein the polarization rotation system comprises a ±

90(2n+1) degree rotator that rotates polarization by ±

90(2n+1) degrees, where n is an integer.

50. The optical imaging system of claim 46, wherein said one or more powered optical elements are substantially optically transmissive to light having a wavelength less than or equal to about 248 nanometers.

51. The optical imaging system of claim 50, wherein said one or more powered optical elements are substantially optically transmissive to light having a wavelength less than or equal to about 193 nanometers.

52. The optical imaging system of claim 51, wherein said one or more powered optical elements are substantially optically transmissive to light having a wavelength less than or equal to about 157 nanometers.

53. The optical imaging system of claim 46, wherein said one or more powered optical elements comprise cubic crystal.

54. The optical imaging system of claim 53, wherein said one or more powered optical elements comprise material selected from the group consisting of calcium fluoride, barium fluoride, lithium fluoride, and strontium fluoride.

55. The optical imaging system of claim 46, wherein at least of said one or more powered optical elements has a surface with an asymmetric variation in curvature.

56. The optical imaging system of claim 55, wherein said surface with asymmetric variation in curvature comprises a toroidal surface.

57. The optical imaging system of claim 56, wherein said surface with asymmetric variation is positioned within said optical imaging system to reduce astigmatism, trefoil aberration, or quadrafoil aberration of said optical system caused by variations in average index of refraction.

58. The optical system as in claim 46, further comprising an additional optical element comprising non-cubic crystalline material and having a surface with an asymmetric variation in curvature.

59. An optical apparatus for transmitting a light, comprising:

a plurality of optical elements having birefringence that introduces retardance to said light; and

a circular retarder having orthogonal circular eigenpolarization states, said circular retarder producing phase delay between said eigenpolarization states substantially equivalent to an odd number of quarter wavelengths of said light.

60. An optical system comprising:

a first optics section for receiving a beam of light having a polarization that is propagating therethrough, said first optics section introducing phase delay between orthogonal polarization states of said beam of light;

a second optics section outputting said beam of light, said second optics section also introducing phase delay between said orthogonal polarization states of said beam of light; and

means for rotating the polarization of said beam to reduce total phase delay between said polarization states of said beam of light output from said optical system.

61. The optical system of claim 60, wherein said phase delay associated with said first optics section is substantially matched to said phase delay associated with said second optics section.