Selection of wavelengths for integrated circuit optical metrology

1. A method of selecting wavelengths for use in optical metrology of an integrated circuit having a nominal profile, the method comprising:

determining one or more termination criteria;

determining one or more selection criteria;

creating a correlation matrix for a set of diffraction spectra measured at optical metrology measurement points, the measurement points corresponding to specified wavelengths, each row and each column corresponding to one of the specified wavelengths, the correlation matrix having as matrix members correlation coefficients calculated from the diffraction spectrum values of the wavelengths corresponding to the row and column of the matrix member;

selecting wavelengths using the set of diffraction spectra for the integrated circuit structure, the correlation matrix, the selection criteria, and a wavelength selection algorithm; and

performing the selecting step until the termination criteria are met.

2. The method of claim 1 wherein the termination criteria for the selection of wavelengths includes testing if a selected-wavelength cost function value is less than or equal to a preset cost function value, the selected-wavelength cost function value calculated by comparing a selected-wavelength spectrum to a full-wavelength spectrum, wherein the selected-wavelength spectrum is a simulated diffraction spectrum of the nominal profile of the integrated circuit structure for a metrology device, the simulated diffraction spectrum determined using only the selected wavelengths, and wherein the full-wavelength spectrum is a simulated diffraction spectrum of the nominal profile of the integrated circuit structure for the metrology device, the simulated diffraction spectrum determined using wavelengths used for the metrology device.

3. The method of claim 1 wherein:

the termination criteria for the selection process includes testing if a selected-wavelength cost function value is less than or equal to a preset cost function value, the selected-wavelength cost function value calculated by comparing a selected-wavelength spectrum to one of the set of diffraction spectra,

wherein the selected-wavelength spectrum is a best match diffraction spectrum from a library of diffraction spectra created for the integrated circuit structure, the selected-wavelength spectrum best match determined using only the selected wavelengths.

4. The method of claim 1 wherein:

the termination criteria for the selection of wavelengths includes testing if a selected-wavelength goodness of fit value is equal to or greater than a preset goodness of fit value, the selected-wavelength goodness of fit value calculated by comparing a selected-wavelength spectrum to a full-wavelength spectrum,

wherein the selected-wavelength spectrum is a simulated diffraction spectrum of the nominal profile of the integrated circuit structure for a metrology device, the simulated diffraction spectrum determined using only the selected wavelengths, and

wherein the full-wavelength spectrum is a simulated diffraction spectrum of the nominal profile of the integrated circuit structure for the metrology device, the simulated diffraction spectrum determined using wavelengths used for the metrology device.

5. The method of claim 1 wherein:

the termination criteria for the selection of wavelengths includes testing if a selected-wavelength goodness of fit value is equal to or greater than a preset goodness of fit value, the selected-wavelength goodness of fit value calculated by comparing a selected-wavelength spectrum to one of the set of diffraction spectra,

wherein the selected-wavelength spectrum is a best match diffraction spectrum from a library of diffraction spectra created for the integrated circuit structure, the selected-wavelength spectrum best match determined using only the selected wavelengths.

6. The method of claim 1 wherein the performing the selecting step until the termination criteria are met includes:

creating a symmetric binary matrix derived from the correlation matrix, the symmetric binary matrix having members with values of one or zero,

wherein the correlation matrix member is set to one if the correlation coefficient is equal to or greater than a correlation threshold, and

wherein the correlation matrix member is set to zero if the correlation coefficient is less than the correlation threshold; and

performing at least one iteration of selecting a wavelength corresponding to a row with the most number of one's in the symmetric binary matrix and setting the row and a corresponding column of the symmetric binary matrix to zeros, the row and column corresponding to the wavelength selected, the iterations of selecting a wavelength continuing until the symmetric binary matrix members are all zeros.

7. The method of claim 1 further comprising:

saving the selected wavelengths, selection process information, and structure identification information,

wherein the selection process information includes termination criteria for the selection of wavelengths and wavelength selection criteria.

8. The method of claim 6 , wherein:

the symmetric binary matrix is the same size as the wavelength correlation matrix; and

each row and each column corresponding to one of the specified wavelengths.

9. The method of claim 1 , wherein the correlation matrix has a first column, a second column, a first row, a second row,

wherein the first row and first column of the correlation matrix correspond to a first wavelength;

wherein the second row and second column of the correlation matrix correspond to a second wavelength;

wherein the matrix element at the second row and first column is the correlation coefficient of the second wavelength to the first wavelength; and

wherein the matrix element at the first row and second column is the correlation coefficient of the first wavelength to the second wavelength.

10. A system for selecting wavelengths for use in optical metrology of an integrated circuit having a nominal profile, the system comprising:

an input device for specifying one or more termination criteria and one or more selection criteria; and

a wavelength selector configured to:

create a correlation matrix for a set of diffraction spectra measured at optical metrology measurement points, the measurement points corresponding to specified wavelengths, each row and each column corresponding to one of the specified wavelengths, the correlation matrix having as matrix members correlation coefficients calculated from the diffraction spectrum values of the wavelengths corresponding to the row and column of the matrix member;

until the termination criteria are met, select wavelengths using the set of diffraction spectra for the integrated circuit structure, the correlation matrix, the selection criteria, and a wavelength selection algorithm.

11. The system of claim 10 further comprising:

a wavelength data store, wherein the selected wavelengths, selection process information, and structure identification information are stored, and wherein the selection process information includes termination criteria for the selection of wavelengths and wavelength selection criteria.

12. The system of claim 10 , wherein the correlation matrix has a first column, a second column, a first row, a second row,

wherein the first row and first column of the correlation matrix correspond to a first wavelength;

wherein the second row and second column of the correlation matrix correspond to a second wavelength;

wherein the matrix element at the second row and first column is the correlation coefficient of the second wavelength to the first wavelength; and

wherein the matrix element at the first row and second column is the correlation coefficient of the first wavelength to the second wavelength.

13. A computer-readable storage medium containing computer executable instructions for selecting wavelengths for use in optical metrology of an integrated circuit having a nominal profile, comprising instructions for:

determining one or more termination criteria;

determining one or more selection criteria;

creating a correlation matrix for a set of diffraction spectra measured at optical metrology measurement points, the measurement points corresponding to specified wavelengths, each row and each column corresponding to one of the specified wavelengths, the correlation matrix having as matrix members correlation coefficients calculated from the diffraction spectrum values of the wavelengths corresponding to the row and column of the matrix member;

selecting wavelengths using the set of diffraction spectra for the integrated circuit structure, the correlation matrix, the selection criteria, and a wavelength selection algorithm; and

performing the selecting step until the termination criteria are met.

14. The computer-readable storage medium of claim 13 wherein the termination criteria for the selection of wavelengths includes testing if a selected-wavelength cost function value is less than or equal to a preset cost function value, the selected-wavelength cost function value calculated by comparing a selected-wavelength spectrum to a full-wavelength spectrum, wherein the selected-wavelength spectrum is a simulated diffraction spectrum of the nominal profile of the integrated circuit structure for a metrology device, the simulated diffraction spectrum determined using only the selected wavelengths, and wherein the full-wavelength spectrum is a simulated diffraction spectrum of the nominal profile of the integrated circuit structure for the metrology device, the simulated diffraction spectrum determined using wavelengths used for the metrology device.

15. The computer-readable storage medium of claim 13 wherein:

the termination criteria for the selection process includes testing if a selected-wavelength cost function value is less than or equal to a preset cost function value, the selected-wavelength cost function value calculated by comparing a selected-wavelength spectrum to one of the set of diffraction spectra,

wherein the selected-wavelength spectrum is a best match diffraction spectrum from a library of diffraction spectra created for the integrated circuit structure, the selected-wavelength spectrum best match determined using only the selected wavelengths.

16. The computer-readable storage medium of claim 13 wherein:

the termination criteria for the selection of wavelengths includes testing if a selected-wavelength goodness of fit value is equal to or greater than a preset goodness of fit value, the selected-wavelength goodness of fit value calculated by comparing a selected-wavelength spectrum to a full-wavelength spectrum,

wherein the selected-wavelength spectrum is a simulated diffraction spectrum of the nominal profile of the integrated circuit structure for a metrology device, the simulated diffraction spectrum determined using only the selected wavelengths, and

wherein the full-wavelength spectrum is a simulated diffraction spectrum of the nominal profile of the integrated circuit structure for the metrology device, the simulated diffraction spectrum determined using wavelengths used for the metrology device.

17. The computer-readable storage medium of claim 13 wherein:

the termination criteria for the selection of wavelengths includes testing if a selected-wavelength goodness of fit value is equal to or greater than a preset goodness of fit value, the selected-wavelength goodness of fit value calculated by comparing a selected-wavelength spectrum to one of the set of diffraction spectra,

wherein the selected-wavelength spectrum is a best match diffraction spectrum from a library of diffraction spectra created for the integrated circuit structure, the selected-wavelength spectrum best match determined using only the selected wavelengths.

18. The computer-readable storage medium of claim 13 wherein the instructions for performing the selecting step until the termination criteria are met includes instructions for:

creating a symmetric binary matrix derived from the correlation matrix, the symmetric binary matrix having members with values of one or zero,

wherein the correlation matrix member is set to one if the correlation coefficient is equal to or greater than a correlation threshold, and

wherein the correlation matrix member is set to zero if the correlation coefficient is less than the correlation threshold; and

performing at least one iteration of selecting a wavelength corresponding to a row with the most number of one's in the symmetric binary matrix and setting the row and a corresponding column of the symmetric binary matrix to zeros, the row and column corresponding to the wavelength selected, the iterations of selecting a wavelength continuing until the symmetric binary matrix members are all zeros.

19. The computer-readable storage medium of claim 18 , wherein:

the symmetric binary matrix is the same size as the wavelength correlation matrix; and

each row and each column corresponding to one of the specified wavelengths.

20. The computer-readable storage medium of claim 13 , wherein the correlation matrix has a first column, a second column, a first row, a second row,

wherein the first row and first column of the correlation matrix correspond to a first wavelength;

wherein the second row and second column of the correlation matrix correspond to a second wavelength;

wherein the matrix element at the second row and first column is the correlation coefficient of the second wavelength to the first wavelength; and

wherein the matrix element at the first row and second column is the correlation coefficient of the first wavelength to the second wavelength.