Heterodyne reflectomer for film thickness monitoring and method for implementing

• US 20060192973A1
• Filed: 02/25/2005
• Published: 08/31/2006
• Est. Priority Date: 02/25/2005
• Status: Active Grant

• Current Assignee: Verity Instruments Incorporated
• Original Assignee: Verity Instruments Incorporated
• Inventors: Harvey, Kenneth C., Kueny, Andrew Weeks, Meloni, Mark A., Aiyer, Arun Ananth
• Granted Patent: US 7,339,682 B2
• Time in Patent Office: Days
• US Class Current: 356/487
• CPC Class Codes: G01B 11/0641 with measurement of polarization

Abstract

The present invention is directed to a heterodyne reflectometer system and method for obtaining highly accurate phase shift information from heterodyned optical signals, from which extremely accurate film depths can be calculated. A linearly polarized light comprised of two linearly polarized components that are orthogonal to each other, with split optical frequencies, is directed toward a film causing one of the optical polarization components to lag behind the other due to an increase in the optical path in the film for that component. A pair of detectors receives the beam reflected from the film layer and produces a measurement signal, and the beam prior to incidence on the film layer and generates a reference signal, respectively. The measurement signal and reference signal are analyzed by a phase detector for phase shift. The detected phase shift is then fed into a thickness calculator for film thickness results. A grating interferometer may be included with the heterodyne reflectometer system with a grating, which diffracts the reflected beam into zeroth- and first-order bands, which are then detected by separate detectors. A detector receives the zeroth-order beam and generates another measurement signal. Another detector receives the first-order beam and generates a grating signal. The measurement signal from the grating and reference signal may be analyzed by a phase detector for phase shift, which is related to the thickness of the film. Conversely, either measurement signal may be analyzed with the grating signal by a phase detector for detecting a grating phase shift. The refractive index for the film may be calculated from grating phase shift and the heterodyne phase shift. The updated refractive index is then used for calculating thickness.

Claims

1. A heterodyne reflectometer for measuring thickness comprising:

an optical light source for producing a split frequency, dual polarized beam;

a beam path diverting optical component for propagating the split frequency, dual polarized beam toward a target at a predetermined angle of incidence;

a first detector for receiving the split frequency, dual polarized beam and generating a reference signal;

a second detector for receiving a reflected split frequency, dual polarized beam from the target and generating a measurement signal;

a phase detector for receiving reference signal and the measurement signal, and detecting a phase shift between said reference and measurement signals; and

a data processor for calculating a thickness relating to the target from the phase shift.

2. The heterodyne reflectometer recited in claim 1, wherein the split frequency, dual polarized beam further comprises:

a first elliptical polarized beam component oscillating at a first frequency; and

a second elliptical polarized beam component oscillating at a second frequency, the first frequency being unique from the second frequency.

3. The heterodyne reflectometer recited in claim 1, wherein the split frequency, dual polarized beam further comprises:

a first linear polarized beam component oscillating at a first frequency; and

a second linear polarized beam component oscillating at a second frequency, the first frequency being unique from the second frequency.

4. The heterodyne reflectometer recited in claim 1, wherein the split frequency, linearly polarized beam further comprises:

an s-polarized beam component oscillating at the first frequency; and

a p-polarized beam component, oscillating at the second frequency, wherein the p-polarized beam component is orthogonal to the s-polarized beam component.

5. The heterodyne reflectometer recited in claim 1, wherein the target is a film.

6. The heterodyne reflectometer recited in claim 1, further comprising:

a polarizing mixer for mixing the reflected a reflected first polarization at the first frequency and a reflected second polarization at the second frequency of the reflected split frequency, dual polarized beam.

7. The heterodyne reflectometer recited in claim 1, further comprising:

phase shift corrector for correcting error in the phase shift, wherein the data processor receives the corrected phase shift and calculates the thickness from the corrected phase shift.

8. The heterodyne reflectometer recited in claim 7, wherein the phase shift corrector produces a corrected phase shift by adjusting the phase shift based on a comparison of a difference between an actual phase shift for known thickness and an expected phase shift for the known thickness.

9. The heterodyne reflectometer recited in claim 1, wherein the predetermined angle of incidence is related to a refractive index for the target.

10. The heterodyne reflectometer recited in claim 1, wherein the predetermined angle of incidence is a predetermined default angle.

11. The heterodyne reflectometer recited in claim 1, wherein the predetermined angle of incidence approximates Brewster'"'"'s angle for the target.

12. The heterodyne reflectometer recited in claim 1, wherein a lower extent of the predetermined angle of incidence is 0 degrees based on one of a target material and an interface below the target being isotropic for the split frequency, dual polarized beam.

13. The heterodyne reflectometer recited in claim 1 further comprises:

a grating for diffracting the reflected split frequency, dual polarized beam as a first-order beam;

a third detector for receiving the first-order beam and generating a grating signal; and

a second phase detector for receiving the grating signal and the measurement signal, and detecting a grating induced phase shift between said grating and measurement signals, wherein the data processor calculates the thickness from the phase shift and the grating induced phase shift.

14. The heterodyne reflectometer recited in claim 8 further comprises:

a grating for diffracting the reflected split frequency, dual polarized beam as a first-order beam;

a third detector for receiving the first-order beam and generating a grating signal;

and a second phase detector for receiving grating signal and the measurement signal, and detecting a grating induced phase shift between said grating and measurement signals, wherein the data processor calculates thickness from the corrected phase shift and the grating induced phase shift.

15. The heterodyne reflectometer recited in claim 14 further comprises:

a grating phase shift corrector for producing a corrected grating induced phase shift by correcting error in the grating induced phase shift, wherein the data processor calculates the thickness from the corrected grating induced phase shift and the corrected phase shift.

16. The heterodyne reflectometer recited in claim 15, wherein the data processor calculates a refractive index for the target from the corrected grating induced phase shift and the corrected phase shift, and calculates the thickness from the refractive index and the corrected phase shift.

17. The heterodyne reflectometer recited in claim 1 further comprising:

a second path diverting optical component for receiving the split frequency, dual polarized beam from the target and propagating the split frequency, dual polarized beam toward the film.

18. The heterodyne reflectometer recited in claim 1, wherein the phase detector further comprises:

a mapping function for fitting the reference signal and the measurement signal to a predetermined form.

19. The heterodyne reflectometer recited in claim 18, wherein the phase detector further comprises:

a signal conditioner for normalizing the reference signal and the measurement signal prior to fitting the signals to the predetermined form.

20. The heterodyne reflectometer recited in claim 1, wherein the phase detector further comprises:

a time interval counter for measuring a time between two corresponding reference points in the reference and the measurement signals.

21. The heterodyne reflectometer recited in claim 1, wherein the phase detector further comprises:

a frequency mixer for creating an output voltage proportional to a phase difference between the reference and the measurement signals.

22. The heterodyne reflectometer recited in claim 21, wherein the phase detector further comprises:

a phase shifter for setting the reference and the measurement signals to an initial phase.

23. The heterodyne reflectometer recited in claim 22, wherein the phase detector further comprises:

a feed back loop for feeding the output voltage to the phase shifter.

24. The heterodyne reflectometer recited in claim 1, wherein the phase detector further comprises:

an oscillator for producing a frequency signal;

a first frequency mixer for creating a first beat signal from the reference and the frequency signals.

a second frequency mixer for creating a second beat signal from the measurement and the frequency signals; and

a time interval counter for measuring a time between two corresponding reference points in the first and second beat signals.

25. The heterodyne reflectometer recited in claim 1, wherein the phase detector further comprises:

a Discrete Fourier Transform (DFT) for determining the phase shift between the reference and measurement signals.

26. The heterodyne reflectometer recited in claim 25, wherein the phase detector further comprises:

at least one digitizer for sampling each of the measurement and reference signals at a predetermined rate based on a heterodyne beat frequency for the measurement and reference signals.

27. The heterodyne reflectometer recited in claim 1, wherein the phase detector further comprises:

a cross correlation function for determining the phase shift between the reference and heterodyne signals.

28. The heterodyne reflectometer recited in claim 27, wherein the phase detector further comprises:

at least one digitizer for sampling each of the measurement and reference signals at a predetermined rate based on a heterodyne beat frequency for the measurement and reference signals and for determining a delay to a number of sample points in a beat cycle for the measurement and reference signals.

29. A heterodyne reflectometer for measuring a thickness parameter comprising:

an optical light source for producing a split frequency, dual polarized beam, said split frequency, dual polarized beam having a first polarized beam component oscillating at a first frequency and a second polarized beam component oscillating at a second frequency, the first frequency being unique from the second frequency;

a beam path diverting optical component for propagating the split frequency, dual polarized beam toward a target at a predetermined angle of incidence, said target having a surface and a body portion;

a first detector for receiving the split frequency, dual polarized beam and generating a reference signal;

a second detector for receiving a reflected split frequency, dual polarized beam from the target and generating a measurement signal, said reflected split frequency, dual polarized beam being comprised of predominately one of a reflected first polarized beam component and a reflected second polarized beam component from the surface of the target and predominately the other of the reflected first polarized beam component and the reflected second polarized beam component from below the surface of the target; and

a phase detector for receiving reference signal and the measurement signal, and detecting a phase shift between said reference and measurement signals, said phase shift being induced by a thickness of said target body.

30. The heterodyne reflectometer recited in claim 29, further comprises:

a data processor for calculating the thickness of said target body thickness from the phase shift.

31. The heterodyne reflectometer recited in claim 29, wherein the first polarization component is a first elliptical polarized beam component and the second polarization component is a second elliptical polarized beam component.

32. The heterodyne reflectometer recited in claim 29, wherein the first polarization component is a first linearly polarized beam component and the second polarization component is a second linearly polarized beam component.

33. The heterodyne reflectometer recited in claim 29, wherein the first polarization component is an s-polarized beam component and the second polarization component is a p-polarized beam component, wherein the p-polarized beam component is orthogonal to the s-polarized beam component.

34. The heterodyne reflectometer recited in claim 29, wherein the target is a film.

35. The heterodyne reflectometer recited in claim 29, wherein the predetermined angle of incidence is related to a refractive index for the target.

36. The heterodyne reflectometer recited in claim 29, wherein the predetermined angle of incidence is a predetermined default angle.

37. The heterodyne reflectometer recited in claim 36, wherein the predetermined default angle is approximately 60 degrees.

38. The heterodyne reflectometer recited in claim 29, wherein the predetermined angle of incidence approximates Brewster'"'"'s angle for the target.

39. The heterodyne reflectometer recited in claim 29, wherein a lower extent of the predetermined angle of incidence is 0 degrees based on one of a target material and an interface below the target being isotropic for the split frequency, dual polarized beam.

40. The heterodyne reflectometer recited in claim 29, further comprising:

a polarizing mixer for mixing the reflected a reflected first polarization at the first frequency and a reflected second polarization at the second frequency of the reflected split frequency, dual polarized beam.

41. The heterodyne reflectometer recited in claim 29, further comprising:

phase shift corrector for correcting error in the phase shift, wherein the data processor receives the corrected phase shift and calculates a thickness from the corrected phase shift.

42. The heterodyne reflectometer recited in claim 41, wherein the phase shift corrector produces a corrected phase shift by adjusting the phase shift based on a comparison of a difference between an actual phase shift for known thickness and an expected phase shift for the known thickness.

43. The heterodyne reflectometer recited in claim 29 further comprises:

a grating for diffracting the reflected split frequency, dual polarized beam as a first-order beam;

a third detector for receiving the first-order beam and generating a grating signal; and

a second phase detector for receiving the grating signal and the measurement signal, and detecting a grating induced phase shift between said grating and measurement signals, wherein the data processor calculates the thickness from the phase shift and the grating induced phase shift.

44. The heterodyne reflectometer recited in claim 42 further comprises:

a grating for diffracting the reflected split frequency, dual polarized beam as a first-order beam;

a third detector for receiving the first-order beam and generating a grating signal;

and a second phase detector for receiving grating signal and the measurement signal, and detecting a grating induced phase shift between said grating and measurement signals, wherein the data processor calculates thickness from the corrected phase shift and the grating induced phase shift.

45. A reflectometry method for measuring a thickness parameter comprising:

directing an optical light source for producing a split frequency, dual polarized beam toward a target at a predetermined angle of incidence, said split frequency, dual polarized beam having a first polarized beam component oscillating at a first frequency and a second polarized beam component oscillating at a second frequency, the first frequency being unique from the second frequency, said target comprising a surface and a body;

generating a reference signal by heterodyning the first polarized beam component oscillating at the first frequency and the second polarized beam component oscillating at the second frequency;

receiving a reflected split frequency, dual polarized beam from the target; and

generating a measurement signal by heterodyning a first reflected polarized beam component oscillating at the first frequency and a second reflected polarized beam component oscillating at the second frequency; and

detecting a phase shift between said reference signal and said measurement signal, said phase shift being induced by a thickness of said target body.

46. The reflectometry method recited in claim 45, further comprises:

calculating the thickness of said target body thickness from the phase shift.

47. The reflectometry method recited in claim 45, wherein the first polarization component is a first elliptical polarized beam component and the second polarization component is a second elliptical polarized beam component.

48. The reflectometry method recited in claim 45, wherein the first polarization component is a first linearly polarized beam component and the second polarization component is a second linearly polarized beam component.

49. The reflectometry method recited in claim 45, wherein the first polarization component is an s-polarized beam component and the second polarization component is a p-polarized beam component, wherein the p-polarized beam component is orthogonal to the s-polarized beam component.

50. The reflectometry method recited in claim 45, wherein the target is a film.

51. The reflectometry method recited in claim 45, wherein the predetermined angle of incidence is related to a refractive index for the target.

52. The reflectometry method recited in claim 45, wherein the predetermined angle of incidence is a predetermined default angle.

53. The reflectometry method recited in claim 52, wherein the predetermined default angle is approximately 60 degrees.

54. The reflectometry method recited in claim 45, wherein the predetermined angle of incidence approximates Brewster'"'"'s angle for the target.

55. The reflectometry method recited in claim 45, wherein a lower extent of the predetermined angle of incidence is 0 degrees based on one of the target body and an interface below the surface of the target being isotropic for the split frequency, dual polarized beam.

56. The reflectometry method recited in claim 45, further comprising:

mixing the reflected a reflected first polarization at the first frequency and a reflected second polarization at the second frequency of the reflected split frequency, dual polarized beam.

57. The reflectometry method recited in claim 45, further comprising:

correcting error in the phase shift; and

calculating an error corrected thickness from the corrected phase shift.

58. The reflectometry method recited in claim 57, wherein correcting error in the phase shift further comprises:

producing a corrected phase shift by adjusting the phase shift based on a comparison of a difference between an actual phase shift for known thickness and an expected phase shift for the known thickness.

59. The reflectometry method recited in claim 45 further comprises:

diffracting the reflected split frequency, dual polarized beam as a first-order beam, said first-order beam comprises a first first-order polarized beam component and a second first-order polarized beam component;

receiving the first-order beam;

generating a grating signal by heterodyning the first first-order polarized beam component and the second first-order polarized beam component;

detecting a grating induced phase shift between said grating and measurement signals; and

calculating a thickness from the phase shift and the grating induced phase shift.

60. The reflectometry method recited in claim 58 further comprises:

diffracting the reflected split frequency, dual polarized beam as a first-order beam, said first-order beam comprises a first first-order polarized beam component and a second first-order polarized beam component;

receiving the first-order beam;

generating a grating signal by heterodyning the first first-order polarized beam component and the second first-order polarized beam component;

detecting a grating induced phase shift between said grating and measurement signals; and

calculating a thickness from the phase shift and the grating induced phase shift.

61. The reflectometry method recited in claim 60 further comprises:

producing a corrected grating induced phase shift by correcting error in the grating induced phase shift based on a comparison between an actual phase shift for known refractive index and an expected phase shift for the known refractive index; and

calculating an error corrected thickness from the corrected grating induced phase shift and the corrected phase shift.

62. The reflectometry method recited in claim 61 further comprises:

calculating a refractive index for the target from the corrected grating induced phase shift and the corrected phase shift; and

calculating an error corrected thickness from the refractive index and the corrected phase shift.