Method and apparatus for accurate calibration of a reflectometer by using a relative reflectance measurement

• US 20070215801A1
• Filed: 04/25/2007
• Published: 09/20/2007
• Est. Priority Date: 08/11/2004
• Status: Active Grant

• Current Assignee: Bruker Technologies Ltd. (Bruker Corporation)
• Original Assignee: Jordan Valley Semiconductors Ltd. (Bruker Corporation)
• Inventors: Walsh, Phillip, Harrison, Dale
• Granted Patent: US 7,663,097 B2
• Time in Patent Office: Days
• US Class Current: 250/252.100
• CPC Class Codes: G01B 11/0625 with measurement of absorption or reflection
• CPC Class Codes: G01N 21/211 Ellipsometry optical thickness measurement G01B11/06
• CPC Class Codes: G01N 21/276 with alternation of sample and standard in optical path
• CPC Class Codes: G01N 21/278 Constitution of standards
• CPC Class Codes: G01N 21/33 using ultraviolet light G01N21/39 takes precedence
• CPC Class Codes: G01N 21/9501 Semiconductor wafers manufacturing processes per se of semiconductor devices implementing a measuring step H01L22/10

Abstract

A reflectometer calibration technique is provided that may include the use of two calibration samples in the calibration process. Further, the technique allows for calibration even in the presence of variations between the actual and assumed properties of at least one or more of the calibration samples. In addition, the technique utilizes a ratio of the measurements from the first and second calibration samples to determine the actual properties of at least one of the calibration samples. The ratio may be a ratio of the intensity reflected from the first and second calibration samples. The samples may exhibit relatively different reflective properties at the desired wavelengths. In such a technique the reflectance data of each sample may then be considered relatively decoupled from the other and actual properties of one or more of the calibration samples may be calculated. The determined actual properties may then be utilized to assist calibration of the reflectometer.

Claims

1. A method of calibrating a reflectometer, comprising:

providing two or more calibration samples, wherein the reflectance properties of the calibration samples differ from one another;

collecting a set of measured data from each of the calibration samples; and

utilizing a combination of the measured data that is independent of the source intensity, I₀, to determine a property of at least one of the calibration samples so that reflectance data from an unknown sample may be calibrated.

2. The method of claim 1, wherein one or more of the calibration samples have one or more contaminant layers, wherein one or more properties of the contaminant layers are determined through analysis of the combination of the measured data.

3. The method of claim 1, wherein the data collected from the calibration samples includes intensity data.

4. The method of claim 3, wherein one or more reflectance ratios are obtained from the intensity data of the calibration samples.

5. The method of claim 4, wherein one or more of the calibration samples have one or more contaminant layers, wherein one or more properties of the contaminant layers are determined through analysis of one or more of the reflectance ratios.

6. The method of claim 4, wherein a source intensity profile is obtained through use of the reflectance ratios and wherein the reflectance of an unknown sample is calibrated by use of the source intensity profile.

7. The method of claim 6, wherein the source intensity profile is obtained by first analyzing the reflectance ratios using thin film models and a regression analysis to adjust one or more properties of one or more of the calibration samples and wherein a result of the analysis is used to derive an absolute reflectance of one or more of the calibration samples, wherein the absolute reflectance is used to obtain the source intensity profile via I₀=I_cal/R_cal, wherein a reflectance of an unknown sample is calibrated by use of the determined source intensity profile via R=I_r/I₀.

8. The method of claim 3, wherein at least one of the calibration samples is expected to exhibit variations from its assumed physical properties.

9. The method of claim 8, wherein an actual reflectance is obtained for at least the one of the calibration samples that is expected to exhibit variations from its assumed physical properties.

10. The method of claim 9, wherein a source intensity profile is obtained through use of the actual reflectance and wherein the reflectance of an unknown sample is calibrated by use of the source intensity profile.

11. The method of claim 1, wherein at least one of the calibration samples is expected to exhibit variations from its assumed physical properties.

12. The method of claim 11, wherein variations are expected in the assumed physical properties of all of the calibration samples.

13. The method of claim 11, wherein an initial source intensity profile is calculated utilizing an assumed reflectance of the at least one of calibration samples that are expected to exhibit variations from its assumed physical properties.

14. The method of claim 13, wherein a recalculated source intensity profile is calculated utilizing a calculated actual reflectance of the at least one calibration samples that are expected to exhibit variations from its assumed physical properties.

15. The method of claim 1, wherein the data from the calibration samples are combined in a manner that decouples the effects of the parameters to be determined.

16. The method of claim 1, wherein three or more calibration samples are provided.

17. The method of claim 16, wherein the three or more calibration samples are comprised of a first calibration sample having a thinner film, a second calibration sample having a first thicker film and a third calibration samples having a second thicker film, the first and second thicker films being distinct films.

18. The method of claim 17, wherein the first calibration sample is comprised of a thin SiO₂ film, the second calibration sample is comprised of a thicker SiO₂ film, and the third calibration sample is comprised of a thicker MgF₂ film.

19. The method of claim 18, wherein the thin SiO₂ film on the first calibration sample is a native oxide.

20. The method of claim 19, wherein one or more contaminant layers are explicitly modeled as part of one or more of the calibration samples, wherein one or more properties of the contaminant layers are determined through use of the combination of measured data.

21. The method of claim 20, where an SiO₂/Si interface layer is explicitly modeled as part of one or more of the calibration samples.

22. The method of claim 21, where the SiO₂/Si interface layer thickness is pre-characterized and held fixed during the calibration.

23. The method of claim 20, where an MgF₂/Si interface layer is explicitly modeled as part of the calibration samples.

24. The method of claim 23, where the MgF₂/Si interface layer thickness is pre-characterized and held fixed during the calibration.

25. The method of claim 16, wherein one or more of the calibration samples have one or more contaminant layers, wherein one or more properties of the contaminant layers are determined through analysis of the combination of the measured data.

26. The method of claim 16, wherein the data collected from the calibration samples includes intensity data.

27. The method of claim 26, wherein one or more reflectance ratios are obtained from the intensity data of the calibration samples.

28. The method of claim 27, wherein one or more of the calibration samples have one or more contaminant layers, wherein one or more properties of the contaminant layers are determined through analysis of one or more of the reflectance ratios.

29. The method of claim 27, wherein a source intensity profile is obtained through use of the reflectance ratios and wherein the reflectance of an unknown sample is calibrated by use of the source intensity profile.

30. The method of claim 29, wherein the source intensity profile is obtained by first analyzing the reflectance ratios using thin film models and a regression analysis to adjust one or more properties of one or more of the calibration samples and wherein a result of the analysis is used to derive an absolute reflectance of one or more of the calibration samples, wherein the absolute reflectance is used to obtain the source intensity profile via I₀=I_cal/R_cal, wherein a reflectance of an unknown sample is calibrated by use of the determined source intensity profile via R=I_r/I₀.

31. The method of claim 27, wherein at least one of the calibration samples is expected to exhibit variations from its assumed physical properties.

32. The method of claim 31, wherein an actual reflectance is obtained for at least the one of the calibration samples that is expected to exhibit variations from its assumed physical properties.

33. The method of claim 32, wherein a source intensity profile is obtained through use of the actual reflectance and wherein the reflectance of an unknown sample is calibrated by use of the source intensity profile.

34. The method of claim 16, wherein at least one of the calibration samples is expected to exhibit variations from its assumed physical properties.

35. The method of claim 34, wherein variations are expected in the assumed physical properties of all of the calibration samples.

36. The method of claim 34, wherein an initial source intensity profile is calculated utilizing an assumed reflectance of the at least one of calibration samples that are expected to exhibit variations from its assumed physical properties.

37. The method of claim 36, wherein a recalculated source intensity profile is calculated utilizing a calculated actual reflectance of the at least one calibration samples that are expected to exhibit variations from its assumed physical properties.

38. The method of claim 16, wherein the data from the calibration samples are combined in a manner that decouples the effects of the parameters to be determined.

39. A method of calibrating a reflectometer which operates at wavelengths that include at least some wavelengths below deep ultra-violet (DUV) wavelengths, comprising:

providing a plurality of calibration samples, wherein the reflectance properties of at least some the calibration samples are different;

collecting data sets from the calibration samples including at least some intensity data collected for wavelengths below DUV wavelengths; and

utilizing a combination of the data sets that is independent of a source intensity I₀ to determine a reflectance of at least one of the calibration samples to assist in calibrating the reflectometer at wavelengths that include at least some wavelengths below DUV wavelengths.

40. The method of claim 39, wherein one or more of the calibration samples have one or more contaminant layers, wherein one or more properties of the contaminant layers are determined through analysis of the combination of data sets.

41. The method of claim 39, wherein reflectance properties of the calibration samples are decoupled from each other such that actual physical properties of at least one of the calibration samples may be calculated based upon the obtained intensity data of the calibration samples.

42. The method of claim 39, wherein:

the combination of the data sets comprises ratios of intensities obtained from the calibration samples,

a source intensity profile is obtained through use of the ratios, and

the reflectance of a unknown sample is calibrated by use of the source intensity profile.

43. The method of claim 39, wherein:

an assumed reflectance of a first one of the calibration samples and the corresponding data set are utilized to calculate an initial source intensity profile,

a reflectance of one or more of the other calibration samples is obtained utilizing the corresponding set of data and the initial source intensity profile,

a ratio of assumed reflectance of the first calibration sample and the obtained reflectance of one or more of the other calibration samples are used to determine an actual property of the first calibration sample, and

a recalculated source intensity profile is obtained utilizing a reflectance of the first calibration sample that is based upon the determined actual property.

44. The method of claim 43, wherein the actual property of the first calibration sample is a material thickness.

45. The method of claim 39, wherein reflectance ratios are obtained from the intensity data of the calibration samples.

46. The method of claim 45, wherein one or more of the calibration samples have one or more contaminant layers, wherein one or more properties of the contaminant layers are determined through analysis of the reflectance ratio.

47. The method of claim 45, wherein a source intensity profile is obtained through use of the reflectance ratios and wherein the reflectance of an unknown sample is calibrated by use of the source intensity profile.

48. The method of claim 45, wherein one or more of the calibration samples are expected to exhibit variations from their assumed physical properties.

49. The method of claim 48, wherein an actual reflectance is obtained for the one or more calibration samples that are expected to exhibit variations from their assumed physical properties.

50. The method of claim 49, wherein a source intensity profile is obtained through use of the actual reflectance and wherein the reflectance of an unknown sample is calibrated by use of the source intensity profile.

51. The method of claim 39, wherein one or more of the calibration samples are expected to exhibit variations from their assumed physical properties.

52. The method of claim 39, wherein three or more calibration samples are provided.

53. The method of claim 52, wherein the three or more calibration samples are comprised of a first calibration sample having a thinner film, a second calibration sample having a first thicker film and a third calibration samples having a second thicker film, the first and second thicker films being distinct films.

54. The method of claim 53, wherein the first calibration sample is comprised of a thin SiO₂ film, the second calibration sample is comprised of a thicker SiO₂ film, and the third calibration sample is comprised of a thicker MgF₂ film.

55. The method of claim 54, wherein the thin SiO₂ film on the first calibration sample is a native oxide.

56. The method of claim 55, wherein one or more contaminant layers are explicitly modeled as part of three or more of the calibration samples, wherein one or more properties of the contaminant layers are determined through use of the combination of the data sets.

57. The method of claim 56, where an SiO₂/Si interface layer is explicitly modeled as part of one or more of the calibration samples.

58. The method of claim 57, where the SiO₂/Si interface layer thickness is pre-characterized and held fixed during the calibrating.

59. The method of claim 56, where an MgF₂/Si interface layer is explicitly modeled as part of the calibration samples.

60. The method of claim 59, where the MgF₂/Si interface layer thickness is pre-characterized and held fixed during the calibrating.

61. The method of claim 39, wherein one or more of the calibration samples have one or more contaminant layers, wherein one or more properties of the contaminant layers are determined through analysis of the combination of data sets.

62. The method of claim 39, wherein reflectance properties of the calibration samples are decoupled from each other such that actual physical properties of at least one of the calibration samples may be calculated based upon the obtained intensity data of the calibration samples.

63. The method of claim 39, wherein:

the combination of the data sets comprises ratios of intensities obtained from the calibration samples,

a source intensity profile is obtained through use of the ratios, and

the reflectance of a unknown sample is calibrated by use of the source intensity profile.

64. The method of claim 39, wherein:

an assumed reflectance of a first one of the calibration samples and the corresponding data set are utilized to calculate an initial source intensity profile,

a reflectance of one or more of the other calibration samples is obtained utilizing the corresponding set of data and the initial source intensity profile,

a ratio of assumed reflectance of the first calibration sample and the obtained reflectance of one or more of the other calibration samples are used to determine an actual property of the first calibration sample, and

a recalculated source intensity profile is obtained utilizing a reflectance of the first calibration sample that is based upon the determined actual property.

65. The method of claim 64, wherein the actual property of the first calibration sample is a material thickness.

66. The method of claim 39, wherein reflectance ratios are obtained from the intensity data of the calibration samples.

67. The method of claim 66, wherein one or more of the calibration samples have one or more contaminant layers, wherein one or more properties of the contaminant layers are determined through analysis of the reflectance ratio.

68. The method of claim 66, wherein a source intensity profile is obtained through use of the reflectance ratios and wherein the reflectance of an unknown sample is calibrated by use of the source intensity profile.

69. The method of claim 66, wherein one or more of the calibration samples are expected to exhibit variations from their assumed physical properties.

70. The method of claim 69, wherein an actual reflectance is obtained for the one or more calibration samples that are expected to exhibit variations from their assumed physical properties.

71. The method of claim 70, wherein a source intensity profile is obtained through use of the actual reflectance and wherein the reflectance of an unknown sample is calibrated by use of the source intensity profile.

72. The method of claim 39, wherein one or more of the calibration samples are expected to exhibit variations from their assumed physical properties.

73. A method of analyzing reflectometer data, comprising:

providing three or more reflectometer samples, wherein the optical response properties of the reflectometer samples are distinct from one another;

collecting optical response data from each of the reflectometer samples; and

determining at least one property of at least one of the reflectometer samples by utilizing the sets of the optical response data in a manner independent of an incident reflectometer intensity that is utilized when collecting the sets of optical response data.

74. The method of claim 73, wherein the property is a variation in a physical property of at least one of the reflectometer samples.

75. The method of claim 74, wherein at least one of the reflectometer samples is a calibration sample.

76. The method of claim 75, wherein all of the reflectometer samples are calibration samples.

77. The method of claim 73, wherein the determining step further comprises:

utilizing ratios of a plurality of combinations of the multiple sets of optical response data.

78. The method of claim 77, wherein the ratios are based on the optical intensity measured from the reflectometer samples.

79. The method of claim 77, wherein utilization of the ratios allows for determination of a change in the at least one property.