Overlay measurements using zero-order cross polarization measurements

1. A method of obtaining overlay measurements for semiconductor wafer, the method comprising:

obtaining zero-order cross polarization measurements of a site on the wafer having a first ridge and a second ridge,

wherein the first ridge was formed using a first mask, and

wherein the second ridge was formed using a second mask; and

determining any overlay error between the first and second masks used to form the first and second ridges based on the obtained zero-order cross polarization measurements.

2. The method of claim 1 , wherein obtaining zero-order cross polarization measurements comprises:

obtaining a first zero-order cross polarization measurement and a second zero-order cross polarization measurement, the first and second zero-order cross polarization measurements having opposite polarizations.

3. The method of claim 2 , wherein determining an overlay error comprises:

comparing the first and second zero-order cross polarization measurements,

wherein an overlay error exists between the first and second masks when there is a difference between the first and second zero-order cross polarization measurements.

4. The method of claim 3 , further comprising:

comparing the difference between the obtained first and second zero-order cross polarization measurements to differences between a first response curve and a second response curve,

wherein the first and second response curves each characterize a relationship between different possible misalignments of the first and second masks and zero-order cross polarization measurements, and

wherein the zero-order cross polarization measurements of the first response curve have opposite polarizations of the zero-order cross polarization measurements of the second response curve.

5. The method of claim 4 further comprising:

comparing the difference between the obtained first and second zero-order cross polarization measurements to a calibration curve,

wherein the calibration curve characterizes a relationship between different possible misalignments of the first and second masks and differences between the first and second response curves.

6. The method of claim 1 , wherein the zero-order cross polarization measurements are obtained using an oblique and conical incident signal.

7. A computer-readable storage medium containing computer executable instructions for causing a computer to obtain overlay measurements for a semiconductor wafer, comprising instructions for:

obtaining zero-order cross polarization measurements of a site on the wafer having a first ridge and a second ridge,

wherein the first ridge was formed using a first mask, and

wherein the second ridge was formed using a second mask; and

determining any overlay error between the first and second masks used to form the first and second ridges based on the obtained zero-order cross polarization measurements.

8. The computer-readable storage medium of claim 7 , wherein obtaining zero-order cross polarization measurements comprises:

obtaining a first zero-order cross polarization measurement and a second zero-order cross polarization measurement, the first and second zero-order cross polarization measurements having opposite polarizations.

9. The computer-readable storage medium of claim 8 , wherein determining any overlay error comprises:

comparing the first and second zero-order cross polarization measurements,

wherein an overlay error exists between the first and second masks when there is a difference between the first and second zero-order cross polarization measurements.

10. The computer-readable storage medium of claim 9 , further comprising:

comparing the difference between the obtained first and second zero-order cross polarization measurements to differences between a first response curve and a second response curve,

wherein the first and second response curves each characterize a relationship between different possible misalignments of the first and second masks and zero-order cross polarization measurements, and

wherein the zero-order cross polarization measurements of the first response curve have opposite polarizations of the zero-order cross polarization measurements of the second response curve.

11. The computer-readable storage medium of claim 10 , further comprising:

comparing the difference between the obtained first and second zero-order cross polarization measurements to a calibration curve,

wherein the calibration curve characterizes a relationship between different possible misalignments of the first and second masks and differences between the first and second response curves.

12. The computer-readable storage medium of claim 7 , wherein the zero-order cross polarization measurements are obtained using an oblique and conical incident signal.

13. A system to obtain overlay measurements of a semiconductor wafer, the system comprising:

an optical metrology system configured to:

obtain a first zero-order cross polarization measurement from a site on the wafer having a first ridge and a second ridge, wherein the first ridge was formed using a first mask, and wherein the second ridge was formed using a second mask;

obtain a second zero-order cross polarization measurement from the same site on the wafer as the first zero-order cross polarization measurement; and

determine any overlay error between the first and second masks used form the first and second ridges based on the obtained first and second zero-order cross polarization measurements.

14. The system of claim 13 , wherein the optical metrology system includes:

a polarizer; and

an analyzer,

wherein the polarizer and the analyzer are set to a first angular setting to obtain the first zero-order cross polarization measurement, and

wherein the polarizer and the analyzer are set to a second angular setting to obtain the second zero-order cross polarization measurement.

15. The system of claim 13 , wherein the optical metrology system includes:

a polarizer having an s-polarizing portion and a p-polarizing portion; and

an analyzer having an s-polarizing portion and a p-polarizing portion,

wherein the first zero-order cross polarization measurement is obtained using the s-polarizing portion of the polarizer and the p-polarizing portion of the analyze, and

wherein the second zero-order cross polarization measurement is obtained using the p-polarizing portion of the polarizer and the s-polarizing portion of the analyze.

16. The system of claim 13 , wherein the optical metrology system includes:

a polarizer having:

an optical path normal to the periodic grating;

a first s-polarizing element;

a second s-polarizing element;

a first p-polarizing element; and

a second p-polarizing element,

wherein the first zero-order cross polarization measurement is obtained using the first s-polarizing element and the first p-polarizing element, and

wherein the second zero-order cross polarization measurement is obtained using the second p-polarizing element and the second s-polarizing element.

17. The system of claim 13 , wherein the first and second zero-order cross polarization measurements having opposite polarizations, and wherein the optical metrology system is configured to:

compare the first and second zero-order cross polarization measurements, and an overlay error exists between the first and second masks when there is a difference between the first and second zero-order cross polarization measurements.

18. The system of claim 17 , wherein the optical metrology system is configured to:

compare the difference between the obtained first and second zero-order cross polarization measurements to differences between a first response curve and a second response curve,

wherein the first and second response curves each characterize a relationship between different possible misalignments of the first and second masks and zero-order cross polarization measurements, and

wherein the zero-order cross polarization measurements of the first response curve have opposite polarizations of the zero-order cross polarization measurements of the second response curve.

19. The system of claim 17 , wherein the optical metrology system is configured to:

compare the difference between the obtained first and second zero-order cross polarization measurements to a calibration curve,

wherein the calibration curve characterizes a relationship between different possible misalignments of the first and second masks and differences between the first and second response curves.

20. The system of claim 13 , wherein the optical metrology system obtains the zero-order cross polarization measurements using an oblique and conical incident signal.