Method and apparatus for performing highly accurate thin film measurements

• US 20070182970A1
• Filed: 08/31/2004
• Published: 08/09/2007
• Est. Priority Date: 08/11/2004
• Status: Active Grant

• Current Assignee: Bruker Technologies Ltd. (Bruker Corporation)
• Original Assignee: Metrosol Inc. (Bruker Corporation)
• Inventors: Harrison, Dale A.
• Granted Patent: US 7,399,975 B2
• Time in Patent Office: Days
• US Class Current: 356/503
• CPC Class Codes: G01B 11/0625 with measurement of absorption or reflection
• CPC Class Codes: G01N 21/274 Calibration, base line adjustment, drift correction
• CPC Class Codes: G01N 21/33 using ultraviolet light G01N21/39 takes precedence
• CPC Class Codes: G01N 21/55 Specular reflectivity

Abstract

A reflectometer data reduction technique is provided that utilizes a ratio of an expected reflectance spectrum of the sample being measured to the actual reflectance spectrum of the sample being measured. The technique is particularly useful in spectral regions that contain sharp spectral features, for example such as the sharp features that thin films often exhibited in the VUV region. In this manner sharp spectral features, for example resulting from either interference or absorption effects are advantageously utilized to better determine a data minimum that is indicative of an actual measurement value. The derivative of this ratio may be utilized to accentuate sharp spectral features. The data reduction techniques may further utilize a two step approach first using a low resolution difference based technique and then a high resolution technique based on reflectance ratios in the region of interest initially identified by the low resolution technique.

Claims

1. A method of measuring a value of a first property of a sample with a reflectometer, comprising:

measuring a reflectance spectrum of the sample, the sample having at least one unknown actual value of an unknown property;

providing an expected reflectance spectrum of the sample utilizing at least one initial assumption regarding the at least one unknown actual value of the unknown property; and

utilizing a ratio between the expected reflectance spectrum and the measured reflectance spectrum to provide an actual value of the first property.

2. The method of claim 1, wherein the first property and the unknown property are the same.

3. The method of claim 2, further comprising:

iteratively adjusting the at least one initial assumption and the expected reflectance spectrum to provide an actual value of the first property.

4. The method of claim 2, further comprising:

utilizing a derivative of the ratio between the expected reflectance spectrum and the measured reflectance spectrum.

5. The method of claim 4, further comprising:

utilizing an integral of the derivative of the ratio between the expected reflectance spectrum and the measured reflectance spectrum.

6. The method of claim 4, wherein an absolute value of the derivative of the ratio between the expected reflectance spectrum and the measured reflectance spectrum is obtained prior to integration.

7. The method of claim 1, wherein the measured reflectance spectrum includes at least a portion of the vacuum ultraviolet (VUV) wavelength region.

8. The method of claim 1, further comprising:

utilizing an amplitude based fitting routine prior to providing an actual value of the first property.

9. The method of claim 8, further comprising:

utilizing the amplitude based fitting routine to provide a coarse measurement and utilizing the ratio between the expected reflectance spectrum and the measured reflectance spectrum to provide a fine measurement of the actual value of the first property.

10. The method of claim 1, further comprising calibrating the reflectometer prior to measuring the reflectance spectrum of the sample, the calibration being according to a calibration routine that comprises:

collecting reflectance data from a standard sample, wherein exact properties of the standard sample vary from assumed properties of the standard sample; and

utilizing, in the wavelength region for which calibration is desired, the presence of significant reflectance variations from the standard sample that result from variations between actual and assumed properties of the standard sample to assist in calibrating the system.

11. The method of claim 1, further comprising calibrating the reflectometer prior to measuring the reflectance spectrum of the sample, the calibration being according to a calibration routine that comprises:

utilizing a standard sample that exhibits sharp spectral features in at least a first wavelength region, calibration being desired in at least a portion of the first wavelength region, the sharp spectral features being a function of the standard sample;

utilizing a first set of reflectance data from the standard sample and to provide a first calibration of the reflectometer based at least in part upon the first set of reflectance data;

utilizing a reference sample that spectrally is relatively featureless in at least the first wavelength region;

utilizing a second set of reflectance data from the reference sample; and

utilizing the first and second set of reflectance data to calibrate the reflectometer.

12. A method for providing data convergence for a reflectometer measurement, comprising:

providing a sample having at least one unknown property;

measuring a reflectance spectrum of the sample, the measurement including a wavelength region in which a sharp spectral feature of the sample is exhibited; and

advantageously utilizing data resulting from the sharp spectral feature exhibited by the sample to determine an actual value of the at least one unknown property.

13. The method of claim 12, wherein the wavelength region includes at least a portion of the VUV region.

14. The method of claim 13, wherein the data resulting from a sharp spectral feature exhibited by the sample is utilized in a fitting routine of which at least a portion is spectral driven fitting routine.

15. The method of claim 14, wherein the spectral driven fitting routine is a ratio based fitting routine.

16. The method of claim 15, wherein the ratio based fitting routine utilizes a ratio of an expected reflectance spectrum of the sample and the measured reflectance spectrum.

17. The method of claim 16, further comprising:

utilizing an amplitude based fitting routine to provide a coarse measurement of an actual value of the at least one unknown property; and

utilizing the ratio based fitting routine to provide a fine measurement of the actual value of the at least one unknown property.

18. The method of claim 16, further comprising calibrating a reflectometer prior to measuring the reflectance spectrum of the sample, the calibration being according to a calibration routine that comprises:

collecting reflectance data from a standard sample, wherein exact properties of the standard sample vary from assumed properties of the standard sample; and

utilizing, in the wavelength region for which calibration is desired, the presence of significant reflectance variations from the standard sample that result from variations between actual and assumed properties of the standard sample to assist in calibrating the system.

19. The method of claim 16, further comprising calibrating a reflectometer prior to measuring the reflectance spectrum of the sample, the calibration being according to a calibration routine that comprises:

utilizing a standard sample that exhibits sharp spectral features in at least a first wavelength region, calibration being desired in at least a portion of the first wavelength region, the sharp spectral features being a function of the standard sample;

utilizing a first set of reflectance data from the standard sample and to provide a first calibration of the reflectometer based at least in part upon the first set of reflectance data;

utilizing a reference sample that spectrally is relatively featureless in at least the first wavelength region;

utilizing a second set of reflectance data from the reference sample; and

utilizing the first and second set of reflectance data to calibrate the reflectometer.

20. A method of measuring at least one property of a thin film material utilizing a reflectance measurement from a sample, comprising:

utilizing an amplitude based fitting routine to provide a low resolution measurement of an actual value of the property; and

utilizing a spectrally driven based fitting routine to provide a high resolution measurement of the actual value of the property.

21. The method of claim 20, wherein the spectrally driven based fitting routine is a ratio based fitting routine.

22. The method of claim 21, wherein the amplitude based fitting routine is a difference based fitting routine.

23. The method of claim 21, wherein the ratio based fitting routine utilizes a ratio between an expected reflectance spectrum and a measured reflectance spectrum.

24. The method of claim 23, wherein at least a portion of the wavelength region of the measured reflectance spectrum exhibits sharp spectral features.

25. A method of measuring an unknown value of a property of a sample with a reflectometer, comprising:

measuring a reflectance spectrum of the sample, at least a portion of the reflectance spectrum including wavelengths below the deep ultraviolet (DUV) region;

providing an expected reflectance spectrum of the sample utilizing at least one initial assumption regarding the unknown value of the property of the sample; and

calculating a ratio between the expected reflectance spectrum and the measured reflectance spectrum;

utilizing a derivative of the ratio between the expected reflectance spectrum and the measured reflectance spectrum;

iteratively adjusting the at least one initial assumption and the expected reflectance spectrum to recalculate the ratio between the expected reflectance spectrum and the measured reflectance spectrum to provide an actual value of the first property.

26. The method of claim 25, further comprising:

integrating a value indicative of the derivative of the ratio between the expected reflectance spectrum and the measured reflectance spectrum.

27. The method of claim 26, wherein an absolute value of the derivative of the ratio between the expected reflectance spectrum and the measured reflectance spectrum is obtained prior to integration.

28. The method of claim 25, wherein the sample exhibits a sharp spectral feature in at least wavelengths below the DUV region.

29. The method of claim 25, further comprising:

utilizing an amplitude based fitting routine to provide a low resolution measurement of an actual value of the property.

30. The method of claim 25, further comprising calibrating the reflectometer prior to measuring the reflectance spectrum of the sample, the calibration being according to a calibration routine that comprises:

collecting reflectance data from a standard sample, wherein exact properties of the standard sample vary from assumed properties of the standard sample; and

utilizing, in the wavelength region for which calibration is desired, the presence of significant reflectance variations from the standard sample that result from variations between actual and assumed properties of the standard sample to assist in calibrating the system.

31. The method of claim 25, further comprising calibrating a reflectometer prior to measuring the reflectance spectrum of the sample, the calibration being according to a calibration routine that comprises:

utilizing a standard sample that exhibits sharp spectral features in at least a first wavelength region, calibration being desired in at least a portion of the first wavelength region, the sharp spectral features being a function of the standard sample;

utilizing a first set of reflectance data from the standard sample and to provide a first calibration of the reflectometer based at least in part upon the first set of reflectance data;

utilizing a reference sample that spectrally is relatively featureless in at least the first wavelength region;

utilizing a second set of reflectance data from the reference sample; and

utilizing the first and second set of reflectance data to calibrate the reflectometer.

32. A reflection measurement apparatus which operates below deep ultra-violet (DUV) wavelengths, the apparatus comprising:

at least one light source, the at least one light source providing a source beam including at least wavelengths below DUV wavelengths;

a spectrometer that receives light, including at least wavelengths below DUV wavelengths, reflected from a sample; and

a data fitting routine coupled to the apparatus in hardware or software, the data fitting routine comprising;

measuring a reflectance spectrum of the sample, the sample having at least one unknown actual value of an unknown property;

providing an expected reflectance spectrum of the sample utilizing at least one initial assumption regarding the at least one unknown actual value of the unknown property; and

utilizing a ratio between the expected reflectance spectrum and the measured reflectance spectrum to provide an actual value of the first property.

33. The apparatus of claim 32, wherein the data fitting routine further comprises:

iteratively adjusting the at least one initial assumption and the expected reflectance spectrum to provide an actual value of the first property.

34. The apparatus of claim 33, wherein the data fitting routine further comprises:

utilizing an amplitude based fitting routine to provide a low resolution measurement of an actual value of the first property; and

utilizing a spectrally driven based fitting routine to provide a high resolution measurement of the actual value of the first property.

35. The apparatus of claim 33, further comprising:

a location in the reflectometer for placement of a standard sample;

a location in the reflectometer for the placement of a reference sample;

, the spectrometer configured to collect reflectance data from the standard sample and the reference sample;

wherein the reflectometer is calibrated by utilizing the presence of significant reflectance variations from the standard sample that result from variations of actual properties of the standard sample from the assumed properties of the standard sample.

36. The reflectometer system of claim 35, wherein the reference sample and the standard sample locations are the same.

37. The reflectometer system of claim 35, wherein the reference sample and standard sample locations are different.

38. The reflectometer system of claim 37, wherein the reference sample is at least one broadband mirror.

39. The reflectometer system of claim 37, wherein the reference sample is comprised of a plurality of optical elements.