Spatial filtering in interferometry

• US 20030211402A1
• Filed: 03/04/2003
• Published: 11/13/2003
• Est. Priority Date: 03/04/2002
• Status: Active Grant

• Current Assignee: Zygo Corporation (Ametek Incorporated)
• Original Assignee: Zygo Corporation (Ametek Incorporated)
• Inventors: Hill, Henry A.
• Granted Patent: US 6,906,784 B2
• Time in Patent Office: Days
• US Class Current: 430/5
• CPC Class Codes: G01B 2290/70 Using polarization in the interferometer
• CPC Class Codes: G01B 9/02007 Two or more frequencies or sources used for interferometric measurement using only beat G01B9/02003
• CPC Class Codes: G01B 9/02018 Multipass interferometers, e.g. double-pass
• CPC Class Codes: G01B 9/02019 contacting different points on same face of object

Abstract

Spatial filtering of beams in interferometry systems is used to reduce a displacement of the beams from an optical path corresponding to the path of the beams in an optimally-aligned system. By reducing beam displacement from the optical path, the system reduces the magnitude of beam shears and associated non-cyclic errors in linear and angular displacements measured by the interferometry systems.

Claims

1. A method, comprising:

directing a first beam and a second beam along respective paths, wherein the first beam contacts a measurement object and changes in the orientation of the measurement object cause a displacement of the first beam from a nominal beam path, and wherein the first and second beams are derived from a common source;

spatial filtering the first beam after it contacts the measurement object, wherein the spatial filtering reduces the displacement of the first beam from the optical path; and

combining the first and second beams to form an output beam, wherein the output beam comprises information about an optical path length difference between the first and second beams.

2. The method of claim 1, wherein the displacement of the first beam from the nominal beam path causes a measurable interferometric phase derived from the output beam to deviate from the expression Φ

=pknL, where p is an integer, k is the wavenumber of the output beam, and nL corresponds to the optical path length difference.

3. The method of claim 2, wherein the deviation comprises a non-cyclic error term that varies in a nonperiodic way on the optical path length difference.

4. The method of claim 1, wherein the nominal path corresponds to the first beam path when the measurement object is in a reference oriented.

5. The method of claim 4, wherein the first beam is normally incident on the measurement object when the measurement object is in the reference orientation.

6. The method of claim 1, wherein the first beam is spatial filtered prior to being combined with the second beam.

7. The method of claim 1, wherein the first beam is spatial filtered after being combined with the second beam.

8. A method, comprising:

directing a first beam and a second beam along respective paths, wherein the first beam contacts a measurement object and wherein the first and second beams are derived from a common source;

spatial filtering the first beam after it contacts the measurement object; and

combining the first and second beams to form an output beam, wherein the output beam comprises information about an optical path length difference between the first and second beams.

9. The method of claim 8, wherein changes in the orientation of the measurement object cause a displacement of the first beam from a nominal beam path.

10. The method of claim 9, wherein the spatial filtering reduces the displacement of the first beam from the nominal beam path.

11. The method of claim 10, wherein the nominal path corresponds to the first beam path when the measurement object is in a reference oriented.

12. The method of claim 11, wherein the first beam is normally incident on the measurement object when the measurement object is in the reference orientation.

13. The method of claim 9, wherein the displacement of the first beam from the nominal beam path causes a measurable interferometric phase derived from the output beam to deviate from the expression Φ

=pknL, where p is an integer, k is the wavenumber of the output beam, and nL corresponds to the optical path length difference.

14. The method of claim 13, wherein the deviation comprises a non-cyclic error term that vanes in a nonperiodic way on the optical path length difference.

15. The method of claim 8, wherein the first and second beams are directed along separate paths by an interferometer.

16. The method of claim 15, wherein the interferometer is a high stability plane mirror interferometer.

17. The method of claim 8, wherein the first beam contacts the measurement object more than once.

18. The method of claim 17, wherein the first beam contacts the measurement object twice.

19. The method of claim 8, wherein the measurement object is a plane mirror.

20. The method of claim 8, wherein the spatial filtering comprises focusing the first beam onto a pinhole aperture.

21. The method of claim 20, wherein the spatial filtering further comprises collimating the focused first beam exiting the pinhole aperture.

22. The method of claim 13, further comprising detecting the measurable interference phase and determining information related to the optical path length difference based on the detected phase.

23. The method of claim 22, wherein the spatial filtering reduces deviations of the phase from the expression Φ

=pknL.

24. The method of claim 22, wherein the spatial filtering reduces the contribution of a non-linear non-cyclic error term to the measurable phase.

25. The method of claim 8, wherein the first beam is spatial filtered prior to being combined with the second beam.

26. The method of claim 8, wherein the first beam is spatial filtered after being combined with the second beam.

27. A method, comprising:

directing a first beam and a second beam along respective paths, wherein the first and second beams are derived from a common source;

combining the first and second beams to form an output beam, wherein the output beam comprises information about an optical path length difference between the first and second beams; and

spatial filtering the first beam, wherein the spatial filtering reduces deviations of a measurable interferometric phase derived from the output beam from the expression Φ

=pknL, where p is an integer, k is the wavenumber of the output beam, and nL corresponds to the optical path length difference.

28. An apparatus, comprising:

an interferometer which during operation directs a first beam and a second beam along respective paths and then combines the first and second beams to produce an output beam, wherein the first beam contacts a measurement object and changes in the orientation of the measurement object cause a displacement of the first beam from a nominal beam path, and wherein the output beam comprises information about an optical path length difference between the first and second beams; and

a spatial filter positioned in the path of the first beam, wherein the spatial filtering reduces the displacement of the first beam from the nominal beam path.

29. The apparatus of claim 28, further comprising a detector positioned to detect an intensity of the output beam.

30. The apparatus of claim 29, further comprising an electronic controller coupled to the detector, which during operation monitors an interference phase related to the optical path length difference between the first and second beams.

31. The apparatus of claim 28, wherein the displacement of the first beam from the nominal beam path causes a measurable interferometric phase derived from the output beam to deviate from the expression Φ

=pknL, where p is an integer, k is the wavenumber of the output beam, and nL corresponds to the optical path length difference.

32. The apparatus of claim 31, wherein the deviation comprises a non-cyclic error term that varies in a nonperiodic way on the optical path length difference.

33. The apparatus of claim 28, wherein the interferometer directs the first beam to contact the measurement object more than once.

34. The apparatus of claim 33, wherein the interferometer directs the first beam to contact the measurement object twice.

35. The apparatus of claim 28, wherein the interferometer is a high stability plane mirror interferometer.

36. The apparatus of claim 28, wherein the spatial filter comprises a focusing lens and a pinhole aperture, wherein during operation the focusing lens focuses the first beam onto the pinhole aperture.

37. The apparatus of claim 36, wherein the spatial filter further comprises a collimating lens, which during operation collimates the focused beam exiting the pinhole aperture.

38. The apparatus of claim 28, wherein the nominal path corresponds to the path of the first beam when the measurement object is in a reference orientation.

39. The apparatus of claim 38, wherein the first beam is normally incident on the measurement object when the measurement object is in the reference orientation.

40. The apparatus of claim 28, wherein the spatial filter is positioned in the path of the output beam.

41. The apparatus of claim 28, further comprising a light source which during operation generates an input beam from which the interferometer derives the first and second beams.

42. A lithography system for use in fabricating integrated circuits on a wafer, the system comprising:

a stage for supporting the wafer;

an illumination system for imaging spatially patterned radiation onto the wafer;

a positioning system for adjusting the position of the stage relative to the imaged radiation; and

the apparatus of claim 28 for monitoring the position of the wafer relative to the imaged radiation.

43. A lithography system for use in fabricating integrated circuits on a wafer, the system comprising:

a stage for supporting the wafer; and

an illumination system including a radiation source, a mask, a positioning system, a lens assembly, and the apparatus of claim 28,

wherein during operation the source directs radiation through the mask to produce spatially patterned radiation, the positioning system adjusts the position of the mask relative to the radiation from the source, the lens assembly images the spatially patterned radiation onto the wafer, and the apparatus monitors the position of the mask relative to the radiation from the source.

44. A beam writing system for use in fabricating a lithography mask, the system comprising:

a source providing a write beam to pattern a substrate;

a stage supporting the substrate;

a beam directing assembly for delivering the write beam to the substrate;

a positioning system for positioning the stage and beam directing assembly relative one another; and

the apparatus of claim 28 for monitoring the position of the stage relative to the beam directing assembly.

52. A method for fabricating integrated circuits, the method comprising using the lithography system of claim 42.

53. A method for fabricating integrated circuits, the method comprising using the lithography system of claim 43.

45. A method for monitoring the position of a measurement object, comprising:

directing a first beam and a second beam along respective paths, wherein the first beam contacts the measurement object and wherein the first and second beams are derived from a common source;

spatial filtering the first beam after it contacts the measurement object;

combining the first and second beams to form an output beam;

detecting a phase of the output beam related to an optical path length difference between the first and second beams; and

monitoring the position of the measurement object based on the detected phase.

46. A lithography method for use in fabricating integrated circuits on a wafer, the method comprising:

supporting the wafer on a moveable stage;

imaging spatially patterned radiation onto the wafer;

adjusting the position of the stage; and

monitoring the position of the stage using the method of claim 45.

47. A lithography method for use in the fabrication of integrated circuits comprising:

directing input radiation through a mask to produce spatially patterned radiation;

positioning the mask relative to the input radiation;

monitoring the position of the mask relative to the input radiation using the method of claim 45;

and

imaging the spatially patterned radiation onto a wafer.

48. A lithography method for fabricating integrated circuits on a wafer comprising:

positioning a first component of a lithography system relative to a second component of a lithography system to expose the wafer to spatially patterned radiation; and

monitoring the position of the first component relative to the second component using the method of claim 45.

49. A method for fabricating integrated circuits, the method comprising the lithography method of claim 46.

50. A method for fabricating integrated circuits, the method comprising the lithography method of claim 47.

51. A method for fabricating integrated circuits, the method comprising the lithography method of claim 48.

54. A method for fabricating a lithography mask, the method comprising:

directing a write beam to a substrate to pattern the substrate;

positioning the substrate relative to the write beam; and

monitoring the position of the substrate relative to the write beam using the method of claim 45.