Generation of a library of periodic grating diffraction signals

1. A computer-readable storage medium containing computer-executable instructions to generate a library of simulated-diffraction signals (simulated signals) of a periodic grating, comprising instructions for;

obtaining a measured-diffraction signal (measured signal) of the periodic grating;

associating hypothetical parameters with a hypothetical profile;

varying the hypothetical parameters within a range to generate a set of hypothetical profiles;

adjusting the range to vary the hypothetical parameters based on the measured signal; and

generating a set of simulated signals from the set of hypothetical profiles; and,

storing the set of simulated signals on the computer-readable storage medium or another computer-readable storage medium.

2. The computer-readable storage medium of claim 1 further comprising instructions for: obtaining extracted optical properties of the periodic grating.

3. The computer-readable storage medium of claim 2 , wherein the periodic grating is formed from a plurality of materials, each material having a refractive index, and wherein extracted optical properties includes extracting real and imaginary parts of the refractive index of each material.

4. The computer-readable storage medium of claim 3 , wherein the real and imaginary parts of the refractive index are extracted using a simulated annealing based optimizer.

5. The computer-readable storage medium of claim 1 further comprising instructions for determining the number of harmonic orders to use in generating the set of simulated signals.

6. The computer-readable storage medium of claim 5 , wherein determining the number of harmonic orders includes running a convergence test.

7. The computer-readable storage medium of claim 6 further comprising instructions for:

generating simulated signals using increasing number of orders;

determining the change in the simulated signals with the increase in the number of orders used; and

selecting the lower number of orders when the change in the simulated signal is less than the minimum change in the measured signal that can be obtained.

8. The computer-readable storage medium of claim 1 further comprising instructions for:

dividing the hypothetical profile into a plurality of hypothetical layers; and

determining the number of hypothetical layers to use in generating the set of simulated signal for the hypothetical profile, wherein each hypothetical profile in the set of hypothetical profiles can be divided into different numbers of hypothetical layers.

9. The computer-readable storage medium of claim 8 , wherein determining the number of hypothetical layers includes:

mapping the determination of the number of hypothetical layers as a set-cover problem; and

solving the set-cover problem.

10. The computer-readable storage medium of claim 1 , wherein the periodic grating includes a first layer formed on a substrate and a second layer formed on the first layer, and wherein the obtaining measured diffraction signal of the periodic grating includes:

obtaining a first-diffraction signal measured after forming the first layer on the substrate prior to forming the second layer on the first layer; and

obtaining a second-diffraction signal measured after forming the second layer on the first layer.

11. The computer-readable storage medium of claim 1 , wherein a plurality of measured signals is obtained from a plurality of sites on a semiconductor wafer.

12. The computer-readable storage medium of claim 11 , wherein a plurality of measured signals is obtained from a plurality of semiconductor wafers.

13. The computer-readable storage medium of claim 11 , wherein adjusting the range to vary the hypothetical parameters includes:

comparing simulated signals and the measured signals using an error metric; and

shifting the range to vary the hypothetical parameters when the simulated signals and the measured signals match and when the hypothetical parameters of the simulated signals lie near an upper or lower limit of the range.

14. The computer-readable storage medium of claim 13 , wherein the error metric is a sum-squared error.

15. The computer-readable storage medium of claim 13 , wherein the error metric is a sum-squared-difference-log error.

16. The computer-readable storage medium of claim 1 further comprising instructions for:

determining a resolution for the set of simulated signals; and

varying the hypothetical parameters used in generating the simulated signals at an increment corresponding to the determined resolution.

17. The computer-readable storage medium of claim 16 , wherein the resolution for the parameters is determined based on a desired critical dimension of the periodic grating.

18. The computer-readable storage medium of claim 17 , wherein determining the resolution for the parameters further comprises:

generating a sub-set of simulated signals including:

a first-simulated signal generated using a first set of hypothetical parameters, wherein the first set of hypothetical parameters includes:

a first-hypothetical parameter associated with the desired critical dimension, and

a second-hypothetical parameter not associated with the desired critical dimension, and

a second-simulated signal generated using a second set of hypothetical parameters, wherein the second set of hypothetical parameters includes:

a first-hypothetical parameter matching the first-hypothetical parameter of the first-simulated signal, and

a second-hypothetical parameter not associated with the desired critical dimension and not matching the second-hypothetical parameter of the first-simulated signal;

removing the second-simulated signal from the sub-set of simulated signals;

comparing the second-simulated signal to the remaining simulated signals in the sub-set of simulated signals; and

reducing the resolution to use for the second-hypothetical parameter in generating the set of simulated signals if the comparison matches the second-simulated signal to the first-simulated signal.

19. The computer-readable storage medium of claim 1 , wherein associating hypothetical parameters with a hypothetical profile further comprises:

determining the number of hypothetical parameters to associate with the hypothetical profile based on the measured signal.

20. The computer-readable storage medium of claim 19 , wherein determining the number of hypothetical parameters further comprises:

generating a set of simulated signals using the determined number of hypothetical parameters;

comparing the measured signal to the set of simulated signals; and

increasing the number of hypothetical parameters if the measured signal fails to match any of the simulated signals in the set of simulated signals.

21. The computer-readable storage medium of claim 19 , wherein determining the number of hypothetical parameters further comprises:

generating a set of simulated signals using the determined number of hypothetical parameters;

comparing the measured signal to the set of simulated signals; and

decreasing the number of hypothetical parameters until the measured signal fails to match any of the simulated signals in the set of simulated signals.

22. The computer-readable storage medium of claim 18 further comprising instructions for:

performing a sensitivity analysis on the hypothetical parameters.

23. A computer-readable storage medium containing computer-executable instructions to generate a library of simulated-diffraction signals (simulated signals) of a periodic grating, comprising instructions for:

obtaining a measured-diffraction signal (measured signal) of the periodic grating;

associating hypothetical parameters with a hypothetical profile;

varying the hypothetical parameters within a range to generate a set of hypothetical profiles;

determining the number of hypothetical parameters to associate with the hypothetical profile based on the measured signal; and

generating a set of simulated signals from the set of hypothetical profiles; and,

storing the set of simulated signals on the computer-readable storage medium or another computer-readable storage medium.

24. The computer-readable storage medium of claim 23 , wherein determining the number of hypothetical parameters further comprises:

generating a set of simulated signals using the determined number of hypothetical parameters;

comparing the measured signal to the set of simulated signals; and

increasing the number of hypothetical parameters if the measured signal fails to match any of the simulated signals in the set of simulated signals.

25. The computer-readable storage medium of claim 23 , wherein determining the number of hypothetical parameters further comprises:

generating a set of simulated signals using the determined number of hypothetical parameters;

comparing the measured signal to the set of simulated signals; and

decreasing the number of hypothetical parameters until the measured signal fails to match any of the simulated signals in the set of simulated signals.

26. The computer-readable storage medium of claim 23 further comprising instructions for:

performing a sensitivity analysis on the hypothetical parameters.

27. The computer-readable storage medium of claim 23 further comprising instructions for:

adjusting the range to vary the hypothetical parameters based on the measured signal.

28. The computer-readable storage medium of claim 27 , wherein a plurality of measured signals are obtained from a plurality of periodic grating, and wherein adjusting the range to the hypothetical parameters includes:

comparing simulated signals and the measured signals using an error metric; and

shifting the range to vary the hypothetical parameters when the simulated signals and the measured signals match and when the hypothetical parameters of the simulated signals lie near an upper or lower limit of the range.

29. The computer-readable storage medium of claim 28 , wherein the error metric is a sum-squared error.

30. The computer-readable storage medium of claim 28 , wherein the error metric is a sum-squared-difference-log error.

31. The computer-readable storage medium of claim 23 further comprising instructions for:

determining a resolution for the set of simulated signals; and

varying the hypothetical parameters used in generating the simulated signals at an increment corresponding to the determined resolution.

32. The computer-readable storage medium of claim 31 , wherein the resolution for the parameters is determined based on a desired critical dimension of the periodic grating.

33. The computer-readable storage medium of claim 32 , wherein determining the resolution for the parameters further comprises:

generating a sub-set of simulated signals including:

a first-simulated signal generated using a first set of hypothetical parameters, wherein the first set of hypothetical parameters includes:

a first-hypothetical parameter associated with the desired critical dimension, and

a second-hypothetical parameter not associated with the desired critical dimension, and

a second-simulated signal generated using a second set of hypothetical parameters, wherein the second set of hypothetical parameters includes:

a first-hypothetical parameter matching the first-hypothetical parameter of the first-simulated signal, and

a second-hypothetical parameter not associated with the desired critical dimension and not matching the second-hypothetical parameter of the first-simulated signal;

removing the second-simulated signal from the sub-set of simulated signals;

comparing the second-simulated signal to the remaining simulated signals in the sub-set of simulated signals; and

reducing the resolution to use for the second-hypothetical parameter in generating the set of simulated signals if the comparison matches the second-simulated signal to the first-simulated signal.

34. The computer-readable storage medium of claim 23 further comprising instructions for:

dividing the hypothetical profile into a plurality of hypothetical layers; and

determining the number of hypothetical layers to use in generating the set of simulated signal for the hypothetical profile, wherein each hypothetical profile in the set of hypothetical profiles can be divided into different numbers of hypothetical layers.

35. The computer-readable storage medium of claim 23 , wherein the periodic grating includes a first layer formed on a substrate and a second layer formed on the first layer, and wherein the obtaining measured diffraction signal of the periodic grating includes:

measuring a first-diffraction signal after forming the first layer on the substrate prior to forming the second layer on the first layer; and

measuring a second-diffraction signal after forming the second layer on the first layer.

36. A system for generating a library of simulated-diffraction signals (simulated signals) of a periodic grating structure, said system comprising:

a signal processor configured to:

obtain a measured-diffraction signal (measured signal) of the periodic grating;

associate hypothetical parameters with a hypothetical profile;

vary the hypothetical parameters within a range to generate a set of hypothetical profiles;

adjust the range to vary the hypothetical parameters based on the measured signal; and

generate a set of simulated signals from the set of hypothetical profiles; and

a library configured to store the generated set of simulated signals and the set of hypothetical profiles.

37. A system for generating a library of simulated-diffraction signals (simulated signals) of a periodic grating structure, said system comprising:

a signal processor configured to:

obtain a measured-diffraction signal (measured signal) of the periodic grating;

associate hypothetical parameters with a hypothetical profile;

vary the hypothetical parameters within a range to generate a set of hypothetical profiles;

determine the number of hypothetical parameters to associate with the hypothetical profile based on the measured signal; and

generate a set of simulated signals from the set of hypothetical profiles; and

a library configured to store the generated set of simulated signals and the set of hypothetical profiles.