Interferometer and method for measuring the refractive index and optical path length effects of air

• US 20020131053A1
• Filed: 03/19/2002
• Published: 09/19/2002
• Est. Priority Date: 02/23/1998
• Status: Active Grant

• Current Assignee: Frank C. Demarest, Henry A. Hill, Peter D.E. Groot
• Original Assignee: Frank C. Demarest, Henry A. Hill, Peter D.E. Groot
• Inventors: Demarest, Frank C., Hill, Henry A., Groot, Peter De
• Granted Patent: US 6,525,826 B2
• Time in Patent Office: Days
• US Class Current: 356/517
• CPC Class Codes: G01B 2290/60 Reference interferometer, i.e. additional interferometer not interacting with object
• CPC Class Codes: G01B 2290/70 Using polarization in the interferometer
• CPC Class Codes: G01B 9/02002 using two or more frequencies
• 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 9/02027 Two or more interferometric channels or interferometers
• CPC Class Codes: G01B 9/02083 characterised by particular signal processing and presentation

Abstract

Apparatus and methods particularly suitable for use in electro-optical metrology and other applications to measure and monitor the refractive index of a gas in a measurement path and/or the change in optical path length of the measurement path due to the gas while the refractive index of the gas may be fluctuating due to turbulence or the like and/or the physical length of the measuring path may be changing. More specifically, the invention employs electronic frequency processing to provide measurements of dispersion of the refractive index, the dispersion being substantially proportional to the density of the gas, and/or measurements of dispersion of the optical path length, the dispersion of the optical path length being related to the dispersion of the refractive index and the physical length of the measurement path. The refractive index of the gas and/or the optical path length effects of the gas are subsequently computed from the measured dispersion of the refractive index and/or the measured dispersion of the optical path length, respectively. The information generated by the inventive apparatus is particularly suitable for use in interferometric distance measuring instruments (DMI) to compensate for errors related to refractive index of gas in a measurement path brought about by environmental effects and turbulence induced by rapid stage slew rates. In preferred embodiments, differential plane mirror interferometer architectures are utilized, the operating wavelengths are approximately harmonically related and may be monitored and/or controlled to meet precision requirements, heterodyne and superheterodyne processing are beneficially used, and phase redundancy is resolved.

Claims

1. Interferometric apparatus for measuring the effects of the refractive index of a gas in a measurement path, said interferometric apparatus comprising:

interferometer means comprising first and second measurement legs, said first and second measurement legs having optical paths structured and arranged such that at least one of them has a variable physical length and at least one of them is at least in part occupied by the gas, the optical path length difference between said first and second measurement legs varying in accordance with the difference between the respective physical lengths of their optical paths and the properties of said gas;

means for generating at least two light beams having different wavelengths;

means for introducing first and second predetermined portions of each of said light beams into said first and second measurement legs, respectively, of said interferometer means so that each of at least one of said first and second predetermined portions of said light beams travels through said first and second measurement legs along predetermined optical paths with the same number of passes, said predetermined first and second portions of said light beams emerging from said interferometer means as exit beams containing information about the respective optical path lengths through said first and second measurement legs at said wavelengths;

means for combining said exit beams to produce mixed optical signals containing information corresponding to the phase differences between each of said exit beams from corresponding ones of said predetermined optical paths of said first and second measurement legs at said wavelengths;

means for detecting said mixed optical signals and generating electrical interference signals containing information corresponding to the effects of the index of refraction of the gas at said different beam wavelengths and the difference in physical path lengths of said measurement legs and their relative rate of change; and

electronic means for analyzing said electrical interference signals to determine the effects of said gas in said measurement leg(s) while compensating for the relative rates at which the physical path lengths of said first and second measurement legs are changing.

2. The interferometric apparatus of claim 1 wherein said electronic means comprises means for directly receiving said electrical interference signals and detecting phases therefrom to generate phase-signals where said phase signals contain information corresponding to the effects of the index of refraction of the gas at said different beam wavelengths and the difference in physical path lengths of said measurement legs and their rates of change.

3. The interferometric apparatus of claim 2 wherein said electronic means further comprises means for resolving phase redundancy in said phase signals.

4. The interferometric apparatus of claim 1 wherein said different wavelengths have an approximate harmonic relationship to each other, said approximate harmonic relationship being expressed as a sequence of ratios, each ratio being comprised of a ratio of low order, non-zero integers.

5. The interferometric apparatus of claim 1 wherein said electronic means further includes phase analyzing means for receiving said electrical interference signals and generating initial electrical phase signals containing information corresponding to the effects of the index of refraction of the gas at said different beam wavelengths and the physical path lengths of said measurement legs occupied by said gas and their rates of change.

6. The interferometric apparatus of claim 5 wherein said electronic means further includes multiplying means for multiplying said initial phase signals by factors proportional to said wavelengths to generate modified phase signals.

7. The interferometric apparatus of claim 6 wherein said electronic means further includes means for receiving said modified phase signals and selectively adding and subtracting them to generate sum and difference phase signals containing information corresponding to the effects of the index of refraction of the gas at said different beam wavelengths and the difference in physical path lengths of said measurement legs o and their relative rates of change.

8. The interferometric apparatus of claim 7 wherein said electronic means further includes means for receiving said sum and difference phase signals and at least one of said initial phase signals to determine the difference in physical lengths, L, of said measurement legs.

9. The interferometric apparatus of claim 8 further including means for resolving redundancies among said initial phase and said sum and difference phase signals.

10. The interferometric apparatus of claim 6 wherein said wavelengths are non-harmonically related.

11. The interferometric apparatus of claim 6 wherein said wavelengths of said light beams have an approximate harmonic relationship to each other, said approximate harmonic relationship being expressed as a sequence of ratios, each ratio being comprised of a ratio of low order non-zero integers.

12. The interferometric apparatus of claim 1 further including a microlithographic means operatively associated with said interferometric apparatus for fabricating integrated circuits on wafers, said microlithographic means comprising:

at least one stage;

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

at least one positioning system for adjusting the position of said at least one stage;

wherein said interferometric apparatus is adapted to measure the position of said at least one stage.

13. The interferometric apparatus of claim 1 further including a microlithographic means operatively associated with said interferometric apparatus for use in fabricating integrated circuits on a wafer, said microlithographic means comprising:

at least one stage for supporting a wafer;

an illumination system including a radiation source, a mask, a positioning system, a lens assembly, and predetermined portions of said interferometric apparatus,

said microlithographic means being operative such that the source directs radiation through said mask to produce spatially patterned radiation, said positioning system adjusts the position of said mask relative to radiation from said source, said lens assembly images said spatially patterned radiation onto the wafer, and said interferometric apparatus measures the position of said mask relative to said radiation from said source.

14. The interferometric apparatus of claim 1 further including microlithographic apparatus operatively associated with said interferometric apparatus for fabricating integrated circuits comprising first and second components, said first and second components being moveable relative to one another, said first first and second components being connected with said first and second measurement legs, moving in concert therewith, such that said interferometric apparatus measures the position of said first component relative to said second component.

15. The interferometric apparatus of claim 1 further including a beam writing system operatively associated with said interferometric apparatus for use in fabricating a lithography mask, said beam writing system comprising:

a source for providing a write beam to pattern a substrate;

at least one stage for supporting a substrate;

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

a positioning system for positioning said at least one stage and said beam directing assembly relative to one another,

said interferometric apparatus being adapted to measure the position of said at least one stage relative to said beam directing assembly.

16. The interferometric apparatus of claim 11 wherein the relative precision of said approximate harmonic relationship expressed as said sequence of ratios is an order of magnitude or more less than the dispersion of the refractive index of said gas times the relative precision, ε

, required for the measurement of the refractivity of said gas or of the change in the difference in optical path lengths of said measurement legs.

17. The interferometric apparatus of claim 1 wherein the relative precision of the relationship between said wavelengths expressed as a ratio(s) relative to a known ratio is greater than a predetermined value corresponding to the precision requirements of a downstream application.

18. The interferometric apparatus of claim 17 further including means for monitoring the relative precision of said relationship expressed as said ratio.

19. The interferometric apparatus of claim 18 further including means responsive to said means for monitoring said relative precision of said relationship expressed as said ratio for providing a feedback signal to control said means for generating said light beams so that said relative precision of said relationship expressed as said ratio is substantially equal to or less than the predetermined value corresponding to the precision requirements of a downstream application.

20. The interferometric apparatus of claim 1 wherein said at least two light beams each have orthogonal polarization states.

21. The interferometric apparatus of claim 20 further including means for introducing a frequency difference(s) between said orthogonal polarization states of said light beams.

22. The interferometric apparatus of claim 21 wherein said means for detecting said mixed optical signals comprises a single photodetector for receiving selected ones of said mixed optical signals beams having predetermined frequency differences.

23. The interferometric apparatus of claim 21 wherein said means for combining said exit beams are adapted to mix said polarization states of said light beams.

24. The interferometric apparatus of claim 23 wherein said information corresponding to said phase differences in said mixed optical signals are phase shifts φ

_j related to the differences in total round-trip physical lengths pL of said measurement legs occupied by said gas according to the formulae

φ

_j=Lpk_jn_j+ζ

_j, j=1,2, . . .

where

k_j=2π

/λ

_j,

p is the number of multiple passes through the at least one of said first and second measurement legs, and n_j are the refractive indices of gas in said at least one of said measurement legs occupied by the gas corresponding to wavelength, λ

_j, and ζ

_j are phase offsets not associated with optical paths.

25. The interferometric apparatus of claim 24 wherein said electrical interference signals comprise heterodyne signals of the form:

s_j=A_jcos[α

_j(t)], j=1,2, . . .

where the time-dependent arguments α

_j(t) are given by

α

_j(t)=2π

ƒ

_jt+φ

_j, j=1,2 . . . .

26. The interferometric apparatus of claim 25 wherein said different wavelengths have an approximate harmonic relationship to each other, said approximate harmonic relationship being expressed as a sequence of ratios, each ratio being comprised of a ratio of low order, non-zero integers.

27. The interferometric apparatus of claim 1 wherein said different wavelengths have a relationship to each other expressed as a ratio, said ratio being non-harmonic.

28. The interferometric apparatus of claim 25 wherein said electronic means is adapted to receive said heterodyne signals and determine said phase shifts,

φ

_j=Lpk_jn_j+ζ

_j, j=1,2, . . . .

29. The interferometric apparatus of claim 26 further including means for generating at least two modified heterodyne signals from said heterodyne signals, said modified heterodyne signals being of the form:

{tilde over (s)}_j=Ã

_jcos[p_jα

_j(t)], j=1,2, . . . ,

where;

α

_j(t)=2π

ƒ

_jt+φ

_j, j=1,2, . . . .

where p_j are non-zero integers where p₁≠

p₂ and have the same ratio as that of said approximate harmonic relationship of said wavelengths.

30. The interferometric apparatus of claim 29 further including means for generating a superheterodyne signal of the form:

31. The interferometric apparatus of claim 26 further including means for generating at least two modified heterodyne signals from said heterodyne signals, said modified heterodyne signals being of the form:

{tilde over (s)}₁′

=Ã

₁′

cos[α

₁(t)/p_2], {tilde over (s)}₂′

=Ã

₂′

cos[α

₂(t)/p₁]

where

α

_j(t)=2π

ƒ

_jt+φ

_j, j=1,2, . . .

and p₁ and p₂ are non-zero integers with p₁≠

p₂ and have the same ratio as that of said approximate harmonic relationship of said wavelengths.

32. The interferometric apparatus of claim 1 wherein said electronic means is further adapted to determine the difference in physical lengths, pL, of said measurement legs occupied by said gas.

33. The apparatus of claim 32 wherein said electronic means is configured to receive the intrinsic optical property, the reciprocal dispersive power, Γ

, of the gas to calculate the difference in physical lengths, L, as;

34. The interferometric apparatus of claim 1 further including a microlithographic means operatively associated with said interferometer means such that said difference in physical lengths, L, may be used to determine the change in relative distance between predetermined elements of said microlithographic means.

35. The interferometric apparatus of claim 1 further including means for aligning said light beams into a single substantially collinear light beam that travels along said at least one of said measurement paths occupied by gas.

36. The interferometric apparatus of claim 1 wherein said interferometer means comprises a polarized Michelson interferometer.

37. The interferometric apparatus of claim 1 wherein said interferometer means comprises a differential plane mirror interferometer.

38. The interferometer apparatus of claim 1 further including means for directly receiving said electrical interference signals from said means for detecting and converting said electrical interference signals to digital form to reduce phase errors in further downstream calculations.

40. The interferometric method of claim 39 wherein said different wavelengths have an approximate harmonic relationship to each other, said approximate harmonic relationship being expressed as a sequence of ratios, each ratio being comprised of a ratio of low order, non-zero integers.

41. The interferometric method of claim 40 wherein the relative precision of said approximate harmonic relationship expressed as said sequence of ratios is an order of magnitude or more less than the dispersion of the refractive index of said gas times the relative precision required for the measurement of the refractivity of said gas or of the change in the differene in optical path lengths of said measurement legs due to said gas.

42. The interferometric method of claim 40 further including the step of monitoring the relative precision of said approximate harmonic relationship expressed as said sequence of ratios.

43. The interferometric method of claim 42 further including the step, responsive to the step of monitoring said relative precision of said approximate harmonic relationship, of providing a feedback signal to control said light beams so that said relative precision of said approximate harmonic relationship is within an order of magnitude or more less than the dispersion of the refractive index of said gas times the relative precision required for the measurement of the refractivity of said gas or of the change in the difference in optical path lengths of said measurement legs due to said gas.

44. The interferometric method of claim 39 wherein said at least two light beams each have orthogonal polarization states.

45. The interferometric method of claim 44 further including the step of introducing a frequency difference(s) between said orthogonal polarization states of said light beams.

46. The interferometric method of claim 45 wherein said step of combining said exit beams comprises mixing said polarization states of said light beams.

47. The interferometric method of claim 46 wherein said information corresponding to said phase differences in said mixed optical signals are phase shifts A related to the differences in total round-trip physical lengths pL of said measurement legs occupied by said gas according to the formulae

φ

_j=Lpk_jn_j+ζ

_j, j=1,2, . . .

where

k_j=2π

/λ

_j,

p is the number of multiple passes through at least one of said first and second measurement legs, and n_j are the refractive indices of gas in said at least one measurement leg corresponding to wavelength, λ

_j.

48. The interferometric method of claim 47 wherein said electrical interference signals comprise heterodyne signals of the form:

s_j=A_jcos[α

_j(t)], j=1,2, . . .

where the time-dependent arguments α

_j(t) are given by

α

_j(t)=2π

ƒ

_jt+φ

_j, j=1,2, . . . .

49. The interferometric method of claim 48 wherein said step of electronically analyzing comprises receiving said heterodyne signals and determining said phase shifts,

φ

_j=Lpk_jn_j+ζ

_j, j=1,2, . . .

50. The interferometric method of claim 48 wherein said wavelengths are harmonically related and further including the step of generating at least two modified heterodyne signals from said heterodyne signals, said modified heterodyne signals being of the form:

{tilde over (s)}_j=Ã

_jcos[p_jα

_j(t)], j=1,2, . . .

where;

α

_j(t)=2π

ƒ

_jt+φ

_j, j=1,2, . . .

and p₁ and p₂ are non-zero integers with p₁≠

p₂ and have the same ratio as that of said approximate harmonic relationship of said wavelengths.

51. The interferometric method of claim 50 further including the step of generating a superheterodyne signal of the form:

52. The interferometric method of claim 48 further including the step of generating at least two modified heterodyne signals from said heterodyne signals, said modified heterodyne signals being of the form:

{tilde over (s)}₁′

=Ã

₁′

cos[α

₁(t)/p₂], {tilde over (s)}₂′

=Ã

₂′

cos[α

₂(t)/p₁]

where

α

_j(t)=2π

ƒ

_jt+φ

_j, j=1,2, . . .

and P₁ and P₂ are non-zero integers with p₁≠

p₂ and have the same ratio as that of said approximate harmonic relationship of said wavelengths.

53. The interferometric method of claim 39 wherein said step of electronically analyzing comprises determining the difference in physical lengths, L, of said measurement legs.

54. The interferometric method of claim 53 wherein said difference in physical lengths, L, may be used to determine the relative distance between predetermined elements of a microlithographic means.

55. The interferometric method of claim 53 wherein said difference in physical lengths, L, is calculated with a predetermined value for the reciprocal relative dispersion, Γ

, of said gas where;

56. The interferometric method of claim 39 further including the step of aligning said light beams into a single substantially collinear light beam that travels along at least one of said measurement legs.

57. The interferometric method of claim 39 wherein said interferometer means comprises a polarized Michelson interferometer.

58. The interferometric method of claim 39 wherein said interferometer means comprises a differential plane mirror interferometer.

59. The interferometer method of claim 39 further including the step of directly receiving said electrical interference signals after their formation to transform them to digital form to reduce phase errors in further downstream calculations.

60. The interferometric method of claim 39 further including the following steps for forming integrated circuits on a wafer:

providing at least one moveable stage;

imaging spatially patterned radiation onto a wafer;

adjusting the position of said at least one stage; and

measuring the position of said at least one stage.

61. The interferometric method of claim 39 further including the steps of:

supporting a wafer on at least one moveable stage;

directing a source of radiation through a mask and lens assembly to produce spatially patterned radiation,

adjusting the position of said mask relative to radiation from said source, said lens assembly imaging said spatially patterned radiation onto the wafer, and measuring the position of said mask relative to said radiation from said source.

62. The interferometric method of claim 39 further including the step of providing a microlithographic apparatus for fabricating integrated circuits comprising first and second components, said first and second components being moveable relative to one another, said first and second components being connected with said first and second measurement legs, respectively, moving in concert therewith, such that the position of said first component relative to said second component is measured.

63. The interferometric method of claim 39 further including the steps of:

providing a pattern of radiation with a write beam source;

supporting a substrate on at least one stage;

directing said write beam such that said pattern of radiation impinges onto the substrate; and

positioning said at least one stage and said write beam of radiation relative to one another, and

measuring the position of said at least one stage relative to said write beam.

64. The interferometric method of claim 39 wherein said wavelengths have a non-harmonic relationship with respect to one another.

65. The interferometric method of claim 39 wherein said step of electronically analyzing further includes the step of receiving said electrical interference signals and extracting the phase therefrom to generate initial electrical phase signals containing information corresponding to the effects of the index of refraction of the gas at said different beam wavelengths and the differnce in physical path lengths of said measurement legs and their relative rates of change.

66. The interferometric method of claim 65 wherein said step of electronically analyzing further includes the step of multiplying said initial phase signals by factors proportional to said wavelengths to generate modified phase signals.

67. The interferometric method of claim 66 wherein said step of electronically analyzing further includes the step of receiving said modified phase signals and selectively adding and subtracting them to generate sum and difference phase signals containing information corresponding to the effects of the index of refraction of the gas at said different beam wavelengths and the differences in round trip physical lengths of said measurement legs and their relative rates of change.

68. The interferometric method of claim 67 wherein said step of electronically analyzing further includes the step of receiving said sum and difference phase signals and at least one of said initial phase signals to determine the difference in round trip physical lengths, L, of said measurement legs.

69. The interferometric method of claim 67 further including the step of resolving redundancies among said initial phase and said sum and difference phase signals.

70. The interferometric method of claim 66 wherein said wavelengths are non-harmonically related.

71. The interferometric method of claim 66 wherein said wavelengths of said light beams have an approximate harmonic relationship to each other, said approximate harmonic relationship being expressed as a sequence of ratios, each ratio being comprised of a ratio of low order non-zero integers.

72. The interferometric method of claim 46 wherein the step of introducing frequency differences between said orthogonal polarization states of said light beams is such that at least two of said light beams have different frequencies between their respective polarization states and so that a single photodetector can be used for generating phase signals from at least two of said exit beams.

39. Interferometric method for measuring the effects of the refractive index of a gas in a measurement path, said interferometric apparatus comprising:

providing an interferometer means comprising a first and second measurement legs, said first and second measurement legs having optical paths structured and arranged such that at least one of them has a variable physical length and at least one of them is at least in part occupied by gas, the optical path length difference between said first and second measurement legs varying in accordance with the difference in the respective physical lengths of their optical paths and the properties of said gas;

generating at least two light beams having different wavelengths;

introducing first and second predetermined portions of each of said light beams into said first and second measurement legs, respectively, of said interferometer means so that each of at least one of said first and second predetermined portions of said light beams travels through said first and second measurement legs along predetermined optical paths with the same number of passes, said predetermined first and second portions of said light beams emerging from said interferometer means as exit beams containing information about the respective optical path lengths through said first and second measurement legs at said wavelengths;

combining said exit beams to produce mixed optical signals containing information corresponding to the phase differences between each of said exit beams from corresponding ones of said predetermined optical paths of said first and second measurement legs at said wavelengths;

detecting said mixed optical signals and generating electrical interference signals containing information corresponding to the effects of the index of refraction of the gas at said different beam wavelengths and the relative physical path lengths between said first and second measurement legs and their relative rates of change; and

electronically analyzing said interference electrical signals to determine the effects of said gas in said measurement leg(s) while compensating for the relative rates at which the physical path lengths of said first and second measurement legs are changing.