Gas insensitive interferometric apparatus and methods

• US 6,330,065 B1
• Filed: 04/28/1999
• Issued: 12/11/2001
• Est. Priority Date: 10/02/1997
• Status: Expired due to Term

• Current Assignee: Zygo Corporation (Ametek Incorporated)
• Original Assignee: Zygo Corporation (Ametek Incorporated)
• Inventors: Hill, Henry Allen
• Primary Examiner(s): Kim, Robert H.
• Assistant Examiner(s): LEE, HWA S
• Application Number:
  US09/301,475
• Time in Patent Office: 958 Days
• Field of Search: 356/450, 356/484, 356/517, 356/491, 356/485, 356/486, 356/487, 356/498, 356/496
• US Class Current: 356/485
• CPC Class Codes: G01B 2290/70 Using polarization in the interferometer
• CPC Class Codes: G01B 9/02018 Multipass interferometers, e.g. double-pass
• CPC Class Codes: G01B 9/02059 Reducing effect of parasitic reflections, e.g. cyclic errors
• CPC Class Codes: G01B 9/02083 characterised by particular signal processing and presentation
• CPC Class Codes: G01N 21/45 using interferometric methods; using Schlieren methods
• CPC Class Codes: G03F 7/70775 Position control, e.g. interferometers or encoders for determining the stage position
• CPC Class Codes: G03F 7/70883 of optical system

Abstract

Displacement measuring interferometers (DMI) are disclosed for use in conjunction with apparatus for measuring and monitoring the intrinsic optical properties of the gas in the measurement leg of a DMI to compensate for variations in the refractive index of the gas that would otherwise render subsequent displacement calculations less accurate. The DMIs may be used for either linear or angular displacements. Cyclic error compensation, wavelength monitoring and correction, and phase redundancy features are included to further enhance the accuracy with which displacement determinations may be made and are particularly suitable for use in photolithographic applications.

Claims

1. A gas insensitive interferometric apparatus for measuring physical displacement, said interferometric apparatus comprising:

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

an interferometer having at least one measurement leg arranged to have a variable physical displacement that is to be measured while occupied by a gas and adapted to receive said light beams and generate non-intrinsic measurement information that varies in accordance with the optical path length and the instantaneous column density of the gas in said measurement leg;

monitor means for determining the intrinsic optical properties of the gas in said measurement leg and generating a monitor signal indicative of said intrinsic optical properties; and

electronic means for receiving said non-intrinsic measurement information and said monitor signal and determining the actual physical displacement in said measurement leg by substantially compensating for the presence of the gas in said measurement leg in accordance with both said non-intrinsic information and said intrinsic optical properties of the gas.

2. The interferometric apparatus of claim 1 wherein said interferometer comprises an amplitude splitting interferometer.

3. The interferometric apparatus of claim 2 wherein said amplitude splitting interferometer further comprises:

a reference leg;

means for dividing said light beams and directing at least a portion of both for travel along both said reference and said measurement legs to generate exit light beams containing information about the respective optical path lengths through said reference and measurement legs at said wavelengths;

means for combining said exit light beams after having traveled along both said reference and measurement leg to generate mixed optical signals containing information corresponding to the phase differences between each of said exit beams that vary in accordance with the optical paths each experienced in traveling along said reference and said measurement legs at both of said wavelengths.

4. The interferometric apparatus of claim 3 further including 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 physical path length of said reference and measurement leg occupied by said gas.

5. The interferometric apparatus of claim 4 further including electronic means for analyzing said electrical interference signals to extract therefrom said non-intrinsic information and combine it with said intrinsic information to determine said displacement.

6. The interferometric apparatus of claim 2 wherein said amplitude splitting interferometer is selected from the group of interferometer forms including the Michelson, Mach-Zehnder, plane mirror, differential plane mirror, and angle compensating.

7. The interferometric apparatus of claim 1 further including means for doubling the frequency of one of said light beams at one of said wavelengths to generate the second of said light beams at the other of said wavelengths.

8. The interferometric apparatus of claim 1 further including means for doubling the frequency of at least one of said exit beams prior to combining said exit beams to produce said mixed optical signals.

9. The interferometric apparatus of claim 1 wherein said electronic means is configured to determine the intrinsic optical property, the reciprocal dispersive power, Γ

, of the gas, as;

$\mathrm{\Gamma <br><br>}=\frac{\left[n_1<br><br>\left({\mathrm{\lambda <br><br>}}_1\right)-1\right]}{\left[n_3<br><br>\left({\mathrm{\lambda <br><br>}}_3\right)-n_2<br><br>\left({\mathrm{\lambda <br><br>}}_2\right)\right]},\text{\hspace{1em}<br><br>}<br><br>\mathrm{and}$

λ

₁, λ

₂, and λ

₃ are wavelengths and n₁, n₂, and n₃ are indices of refraction and wherein the denominator may be replaced by [n₃(λ

₃)−

n₁(λ

₁)] or [n₂(λ

₂)−

n₁(λ

₁)].

10. The interferometric apparatus of claim 1 wherein said electronic means is configured to determine the refractivities of the gas corresponding to each light beam wavelength.

11. The interferometric apparatus of claim 1 wherein said electronic means is configured and arranged to calculate the intrinsic optical property, the reciprocal dispersive power, Γ

, as;

$\mathrm{\Gamma <br><br>}=\frac{\left[n_i<br><br>\left({\mathrm{\lambda <br><br>}}_i\right)-1\right]}{\left[n_{j+1}<br><br>\left({\mathrm{\lambda <br><br>}}_{j+1}\right)-1\right]-\left[n_j<br><br>\left({\mathrm{\lambda <br><br>}}_j\right)-1\right]},\text{\hspace{1em}<br><br>}<br><br>\mathrm{where}$

i and j are integers corresponding to wavelengths.

12. The interferometric apparatus of claim 1 wherein said electronic means is configured to determine the intrinsic optical property, the relative refractivities at different beam wavelengths, where said relative refractivities are of the form:

$\frac{n_{{\mathrm{\lambda <br><br>}}_i-1}}{n_{{\mathrm{\lambda <br><br>}}_j-1}}$

where i and j are integers corresponding to wavelengths and are different from one another.

13. The interferometric apparatus of claim 1 wherein said electronic means further includes means for compensating for cyclic errors present in said non-intrinsic information corresponding to the dispersion of the gas, (n_λ

_j−

n<sub>λ

i</sub>), in said measurement leg, where i and j are integers corresponding to wavelengths and different from one another.

14. The interferometric apparatus of claim 1 wherein said electronic means further includes means for compensating for cyclic errors present in at least one of said electrical interference signals.

15. The interferometric apparatus of claim 1 wherein said electronic means further includes means for compensating for cyclic errors in present in said non-intrinsic information corresponding to the dispersion of the gas, (n_λ

_j−

n<sub>λ

i</sub>), in said measurement leg, where i and j are integers corresponding to wavelengths and different from one another and in at least one of said electrical interference signals.

16. The interferometric apparatus of claim 15 further including means for measuring the accuracy of said wavelengths and generating a wavelength accuracy signal indicative thereof.

17. The interferometric apparatus of claim 16 wherein said electronic means further includes means for receiving said wavelength accuracy signal and using its value in the determination of said actual physical displacement.

18. The interferometric apparatus of claim 16 further including wavelength correction means for receiving said wavelength accuracy signal and generating a control signal to adjust said means for generating said light beams so that said wavelengths thereof are within predetermined limits of accuracy.

19. The interferometric apparatus of claim 1 wherein said interferometer further comprises:

a reference leg and a beam steering assembly having a beam steering element and a positioning system to orient said beam steering element, said beam steering element being arranged to direct at least one of a reference and measurement beam associated, respectively, with said reference and measurement legs, and in contact with said beam steering element, and

a control circuit which, during operation, causes said positioning system to reorient said beam steering element in response to changes in at least one of angular orientation and position of a measurement object.

20. The interferometric apparatus of claim 1 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.

21. The interferometric apparatus of claim 20 wherein said interferometer comprises means for generating multiple passes along at least said measurement leg for said light beams where the number of passes for said light beams are harmonically related in a relationship which is substantially the same as said substantially harmonic relationship between said wavelengths.

22. The interferometric apparatus of claim 21 wherein said means for generating at least two light beams further includes means for generating orthogonally polarized components for each of said light beams.

23. The interferometric apparatus of claim 22 further including means for separating said light beams into pairs of orthogonally polarized components of common wavelength.

24. The interferometric apparatus of claim 23 further including means for spatially separating said orthogonally polarized pairs of components for subsequent downstream use in said interferometer means.

25. The interferometric apparatus of claim 20 wherein the relative precision of relationship of said wavelengths, expressed as said sequence of ratios, is an order of magnitude or more less than the dispersion of the refractive index of said gas, (n₂−

n₁) where n₁ and n₂ are, respectively, the indices of refraction of said gas at said different wavelengths, times the relative precision, ε

, desired for the measurement of the refractivity (n₁−

1) of the gas or of the change in the difference in optical path lengths of said measurement legs due to the gas.

26. The interferometric apparatus of claim 25 further including means for monitoring said relative precision of said approximate harmonic relationship expressed as said sequence of ratios.

27. The interferometric apparatus of claim 25 further including means responsive to said means for monitoring said relative precision of said approximate harmonic relationship for providing a feedback signal to control said means for generating said light beams so that said relative precision of said approximate harmonic relationship is of an order of magnitude or more less than the dispersion of the refractive index of said gas times the relative precision ε

desired for the measurement of the refractivity (n₁−

1) of the gas or of the change in the difference in optical path lengths of said measurement legs due to the gas.

28. The interferometric apparatus of claim 4 further including means for introducing a frequency difference between at least a first and second portion of each of said light beams to generate a set of frequency shifted light beams such that no two beams of said set of frequency shifted light beams have the same frequency.

29. The interferometric apparatus of claim 28 wherein said electrical interference signals comprise heterodyne electrical signals.

30. The interferometric apparatus of claim 4 wherein said electronic means further includes means for receiving said electrical interference signals and directly extracting therefrom phase information corresponding to the select intrinsic optical properties of the gas.

31. 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.

32. The interferometric apparatus of claim 31 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.

33. The interferometric apparatus of claim 32 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.

34. The interferometric apparatus of claim 1 wherein said interferometer has a reference leg that is structured and arranged with said measurement leg so that beams at one of said wavelengths of said light beams travel through at least one of said reference and second measurement legs along predetermined optical paths a different number of passes than beams at the other of said wavelengths to compensate for the relative rates at which the physical path lengths of said reference and second measurement legs are changing.

35. The interferometric apparatus of claim 1 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.

36. The interferometric apparatus of claim 35 wherein said interferometer comprises means for generating multiple passes along at least one said measurement leg for said light beams where the number of passes for said light beams are harmonically related in a relationship which is substantially the same as said substantially harmonic relationship between said wavelengths.

37. 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.

38. 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.

39. 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.

40. The interferometric apparatus of claim 1 monitor means for determining said intrinsic optical properties is located proximate said measurement leg of said interferometer.

41. The interferometric apparatus of claim 1 monitor means for determining said intrinsic optical properties is located proximate to and upstream of said reference leg of said interferometer so as to capture changes in the upstream composition and environmental conditions of the gas prior to the gas reaching said measurement leg.

97. 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 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.

42. The interferometric apparatus of 41 further including means for periodically sampling the values of said intrinsic optical properties to assess any changes in them and update the values of said intrinsic optical properties for use in subsequent calculations should the changes exceed predetermined values.

43. Interferometric apparatus for measuring distances occupied by a gas whose optical properties may vary over the measured distance, said system 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 and one of them may at least in part be occupied by a predetermined medium, 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 and said predetermined medium;

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 said first and second predetermined portions of said light beams travels through said first and second measurement legs along predetermined optical paths, 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 physical path length of said first and second measurement legs occupied by said gas;

means for measuring intrinsic optical properties of the gas and generating corrective information for compensating for the variable optical properties of the gas in said first and second measurement legs; and

electronic means for determining difference in physical length between said first and second reference legs based on the information contained in said interference signals and said corrective information.

44. The interferometric apparatus of claim 43 wherein said interferometer means is selected from the forms comprising the Michelson, the Mach-Zehnder, the plane mirror, the differential plane mirror, and the angle compensating.

45. The interferometric apparatus of claim 43 wherein said interferometer means, said beam wavelengths, and said electronic means are configured and arranged with respect to one another to compensate for Doppler shifts introduced by stage motion.

46. The interferometric apparatus of claim 43 further including means for compensating for cyclic errors in at least one of said electrical interference signals and the dispersion of the gas.

47. The interferometric apparatus of claim 43 wherein said means for measuring intrinsic optical properties comprises and interferometer selected from the forms comprising the Michelson, the Mach-Zehnder, the plane mirror, the differential plane mirror, and the angle compensating.

48. The interferometric apparatus of claim 43 wherein said means for measuring intrinsic optical properties uses portions of the same beams as are used by said interferometer means.

49. The interferometric apparatus of claim 43 further including photolithographic apparatus.

50. A gas insensitive interferometric apparatus for measuring physical length, said interferometric apparatus comprising:

an interferometer having a measurement leg of variable physical length that is to be measured while occupied by a gas, said interferometer being adapted to generate at least one signal indicative of the optical path length of said measurement leg and at least one other signal indicative of the non-intrinsic optical properties of the gas in said measurement leg;

a monitor for measuring the intrinsic optical properties of the gas and generating a corrective signal indicative of said intrinsic optical properties; and

means for receiving said at least one signal and said at least one other signal and determining the actual physical length of said measurement leg by substantially compensating for the presence of the gas in said measurement leg.

51. A gas insensitive interferometric apparatus for measuring variable physical length in a gas, said interferometer comprising:

first interferometer means comprising a reference leg and a measurement leg whose physical length is variable and is occupied by the gas;

second interferometer means for use in the compensation of the presence of the gas in said first interferometer means, said second interferometer means comprising a reference leg and a measurement leg each of which has a predetermined physical path length, said reference leg being configured and arranged to be occupied by a predetermined medium and said measurement leg being configured and arranged to be occupied by the gas;

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

means for introducing at least a portion of each of predetermined ones of said light beams into preselected ones of said reference and measurement legs of said first and second interferometers to generate optical signals that contain information corresponding to;

(a) the optical path length in the measurement path of said first interferometer means at a first one of said wavelengths,

(b) the optical path length in the measurement leg of said first interferometer means at at least one other one of said wavelengths, and

(c) intrinsic optical properties of the gas in said measurement leg of said second interferometer means at first and said at least one other one of said wavelengths;

means for converting said optical signals to electrical signals; and

electronic means for processing said electrical signals to compensate for the presence of the gas in the measurement leg of said first interferometer and determine the physical path length of said measurement leg of said first interferometer means by substantially correcting for the presence of the gas in said measurement leg of said first interferometer means.

52. A gas insensitive interferometric apparatus comprising:

an interferometer having reference and measurement legs where said measurement leg is configured so that the physical path length, L, thereof can vary and is occupied by the gas;

an interferometric Γ

-monitor;

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

means for introducing at least a portion of predetermined ones of each of said light beams into said interferometer and said interferometric Γ

-monitor and generating;

(a) a first signal for said physical path length, L, where L=L₁−

Γ

(L₂−

L₁), L₁ is the optical path length of said measurement leg divided by p₁k₁ where k₁ is the wavenumber and p₁ the number of passes through said measurement leg at λ

₁, L₂ is the optical path length of said measurement leg divided by p₂k₂ where k₂ is the wavenumber and p₂ the number of passes through said measurement leg at wavelength λ

₂, and Γ

=(n₁−

1)/(n₂−

n₁) where n₁ and n₂ are, respectively, the indices of refraction of the gas in said measurement leg at λ

₁ and λ

₂, and

(b) a second signal containing information for calculating Γ

to correct said first signal for errors in said first signal related to the presence of the gas in said measurement leg at λ

₁; and

signal processing means for receiving said first and second signals and calculating Γ and

then said actual physical length, L, of said measurement leg.

53. A gas insensitive interferometric method for measuring physical displacement, said interferometric method comprising the steps of:

generating at least two light beams having different wavelengths;

providing an interferometer having at least one measurement leg arranged to have a variable physical displacement that is to be measured while occupied by a gas and adapted to receive said light beams and generate non-intrinsic measurement information that varies in accordance with the optical path length and the instantaneous column density of the gas in said measurement leg;

determining the intrinsic optical properties of the gas in said measurement leg and generating a monitor signal indicative of said intrinsic optical properties; and

receiving said non-intrinsic measurement information and said monitor signal and determining the actual physical displacement in said measurement leg by substantially compensating for the presence of the gas in said measurement leg in accordance with both said non-intrinsic information and said intrinsic optical properties of the gas.

54. The interferometric method of claim 53 wherein said interferometer comprises an amplitude splitting interferometer.

55. The interferometric method of claim 54 wherein said amplitude splitting interferometer further comprises:

a reference leg;

means for dividing said light beams and directing at least a portion of both for travel along both said reference and said measurement legs to generate exit light beams containing information about the respective optical path lengths through said reference and measurement legs at said wavelengths;

means for combining said exit light beams after having traveled along both said reference and measurement leg to generate mixed optical signals containing information corresponding to the phase differences between each of said exit beams that vary in accordance with the optical paths each experienced in traveling along said reference and said measurement legs at both of said wavelengths.

56. The interferometric method of claim 55 further including the step of 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 physical path length of said reference and measurement leg occupied by said gas.

57. The interferometric method of claim 56 further including the step of analyzing said electrical interference signals to extract therefrom said non-intrinsic information and combine it with said intrinsic information to determine said displacement.

58. The interferometric method of claim 54 wherein said amplitude splitting interferometer is selected from the group of interferometer forms including the Michelson, Mach-Zehnder, plane mirror, differential plane mirror, and angle compensating.

59. The interferometric method of claim 53 further including the step of doubling the frequency of one of said light beams at one of said wavelengths to generate the second of said light beams at the other of said wavelengths.

60. The interferometric method of claim 53 further including the step of doubling the frequency of at least one of said exit beams prior to combining said exit beams to produce said mixed optical signals.

61. The interferometric method of claim 53 determining the intrinsic optical property, the reciprocal dispersive power, Γ

, of the gas, as;

$\mathrm{\Gamma <br><br>}=\frac{\left[n_1<br><br>\left({\mathrm{\lambda <br><br>}}_1\right)-1\right]}{\left[n_3<br><br>\left({\mathrm{\lambda <br><br>}}_3\right)-n_2<br><br>\left({\mathrm{\lambda <br><br>}}_2\right)\right]},\text{\hspace{1em}<br><br>}<br><br>\mathrm{and}$

λ

₁, λ

₂, and λ

₃ are wavelengths and n₁, n₂, and n₃ are indices of refraction and wherein the denominator may be replaced by [n₃(λ

₃)−

n₁(λ

₁)] or [n₂(λ

₂)−

n₁(λ

₁)].

62. The interferometric method of claim 53 wherein said determining the refractivities of the gas corresponding to each light beam wavelength.

63. The interferometric method of claim 53 including the step of calculating the intrinsic optical property, the reciprocal dispersive power, Γ

, as;

$\mathrm{\Gamma <br><br>}=\frac{\left[n_i<br><br>\left({\mathrm{\lambda <br><br>}}_i\right)-1\right]}{\left[n_{j+1}<br><br>\left({\mathrm{\lambda <br><br>}}_{j+1}\right)-1\right]-\left[n_j<br><br>\left({\mathrm{\lambda <br><br>}}_j\right)-1\right]},\text{\hspace{1em}<br><br>}<br><br>\mathrm{where}$

i and j are integers corresponding to wavelengths.

64. The interferometric method of claim 53 determining the intrinsic optical property, the relative refractivities at different beam wavelengths, where said relative refractivities are of the form:

$\frac{n_{{\mathrm{\lambda <br><br>}}_i-1}}{n_{{\mathrm{\lambda <br><br>}}_j-1}}$

where i and j are integers corresponding to wavelengths and are different from one another.

65. The interferometric method of claim 53 further includes the step of compensating for cyclic errors present in said non-intrinsic information corresponding to the dispersion of the gas, (n_λ

_j−

n<sub>λ

i</sub>), in said measurement leg, where i and j are integers corresponding to wavelengths and different from one another.

66. The interferometric method of claim 53 further including the step of compensating for cyclic errors present in at least one of said electrical interference signals.

67. The interferometric method of claim 53 further including the step of compensating for cyclic errors in present in said non-intrinsic information corresponding to the dispersion of the gas, (n_λ

_j−

n<sub>λ

i</sub>), in said measurement leg, where i and j are integers corresponding to wavelengths and different from one another and in at least one of said electrical interference signals.

68. The interferometric method of claim 67 further the step of measuring the accuracy of said wavelengths and generating a wavelength accuracy signal indicative thereof.

69. The interferometric method of claim 68 further including the step of receiving said wavelength accuracy signal and using its value in the determination of said actual physical displacement.

70. The interferometric method of claim 68 further including the step of receiving said wavelength accuracy signal and generating a control signal to adjust said means for generating said light beams so that said wavelengths thereof are within predetermined limits of accuracy.

71. The interferometric method of claim 53 wherein said interferometer further comprises:

a reference leg and a beam steering assembly having a beam steering element and a positioning system to orient said beam steering element, said beam steering element being arranged to direct at least one of a reference and measurement beam associated, respectively, with said reference and measurement legs, and in contact with said beam steering element, and

a control circuit which, during operation, causes said positioning system to reorient said beam steering element in response to changes in at least one of angular orientation and position of a measurement object.

72. The interferometric method of claim 53 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.

73. The interferometric method of claim 72 further including the step of generating multiple passes along at least said measurement leg for said light beams where the number of passes for said light beams are harmonically related in a relationship which is substantially the same as said substantially harmonic relationship between said wavelengths.

74. The interferometric method of claim 73 including the step of generating at least two light beams further includes the step of generating orthogonally polarized components for each of said light beams.

75. The interferometric method of claim 74 further including the step of separating said light beams into pairs of orthogonally polarized components of common wavelength.

76. The interferometric method of claim 75 further includes spatially separating said orthogonally polarized pairs of components for subsequent downstream use in said interferometer means.

77. The interferometric method of claim 72 wherein the relative precision of relationship of said wavelengths, expressed as said sequence of ratios, is an order of magnitude or more less than the dispersion of the refractive index of said gas, (n₂−

n₁) where n₁ and n₂ are, respectively, the indices of refraction of said gas at said different wavelengths, times the relative precision, ε

, desired for the measurement of the refractivity (n₁−

1) of the gas or of the change in the difference in optical path lengths of said measurement legs due to the gas.

78. The interferometric method of claim 77 further including monitoring said relative precision of said approximate harmonic relationship expressed as said sequence of ratios.

79. The interferometric method of claim 78 further the step of, responsive to monitoring said relative precision of said approximate harmonic relationship, providing a feedback signal to control said light beams so that said relative precision of said approximate harmonic relationship is of an order of magnitude or more less than the dispersion of the refractive index of said gas times the relative precision ε

desired for the measurement of the refractivity (n₁−

1) of the gas or of the change in the difference in optical path lengths of said measurement legs due to the gas.

80. The interferometric method of claim 56 further including the step of introducing a frequency difference between at least a first and second portion of each of said light beams to generate a set of frequency shifted light beams such that no two beams of said set of frequency shifted light beams have the same frequency.

81. The interferometric method of claim 80 wherein said electrical interference signals comprise heterodyne electrical signals.

82. The interferometric method of claim 56 wherein said further including receiving said electrical interference signals and directly extracting therefrom phase information corresponding to the select intrinsic optical properties of the gas.

83. The interferometric method of claim 53 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.

84. The interferometric apparatus of claim 83 further including the step of receiving and analyzing the phase of 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.

85. The interferometric method of claim 84 further including the step of multiplying said initial phase signals by factors proportional to said wavelengths to generate modified phase signals.

86. The interferometric method of claim 53 wherein said interferometer has a reference leg that is structured and arranged with said measurement leg so that beams at one of said wavelengths of said light beams travel through at least one of said reference and second measurement legs along predetermined optical paths a different number of passes than beams at the other of said wavelengths to compensate for the relative rates at which the physical path lengths of said reference and second measurement legs are changing.

87. The interferometric method of claim 53 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.

88. The interferometric method of claim 87 including the step of generating multiple passes along at least one said measurement leg for said light beams where the number of passes for said light beams are harmonically related in a relationship which is substantially the same as said substantially harmonic relationship between said wavelengths.

89. The interferometric method of claim 53 further including a microlithographic method operatively associated with said interferometric method for fabricating integrated circuits on wafers, said microlithographic method comprising the steps of:

supporting a wafer on at least one stage;

imaging spatially patterned radiation onto the wafer; and

adjusting the position of said at least one stage;

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

90. The interferometric method of claim 53 further including a microlithographic method operatively associated with said interferometric apparatus for use in fabricating integrated circuits on a wafer, said microlithographic method comprising the steps of:

supporting a wafer on at least one stage;

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

directing radiation through said mask to produce spatially patterned radiation, said positioning system 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.

91. The interferometric method of claim 53 further including a microlithographic method 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 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.

92. The interferometric method of claim 53 further including a beam writing method operatively associated with said interferometric method for use in fabricating a lithography mask, said beam writing method comprising the steps of:

providing a write beam to pattern a substrate;

supporting a substrate on at least one stage;

directing a beam to the substrate; and

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

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

93. The interferometric method of claim 53 wherein said intrinsic optical properties are determined proximate said measurement leg of said interferometer.

94. The interferometric method of claim 53 said intrinsic optical properties are monitored proximate to and upstream of said reference leg of said interferometer so as to capture changes in the upstream composition and environmental conditions of the gas prior to the gas reaching said measurement leg.

95. The interferometric method of claim 93 further including the step of periodically sampling the values of said intrinsic optical properties to assess any changes in them and update the values of said intrinsic optical properties for use in subsequent calculations should the changes exceed predetermined values.

96. The interferometric method of claim 53 further including the step of resolving phase redundancies in said electrical interference signals.