IMAGE BASED OVERLAY MEASUREMENT WITH FINITE GRATINGS

• US 20130208279A1
• Filed: 02/13/2013
• Published: 08/15/2013
• Est. Priority Date: 02/15/2012
• Status: Active Grant

• Current Assignee: Onto Innovation Inc.
• Original Assignee: Nanometrics Incorporated (Onto Innovation Inc.)
• Inventors: Smith, Nigel P.
• Granted Patent: US 10,107,621 B2
• Time in Patent Office: Days
• US Class Current: 356/401
• CPC Class Codes: G01B 11/26 for measuring angles or tapers; for testing the alignment of axes
• CPC Class Codes: G03F 7/70633 Overlay, i.e. relative alignment between patterns printed by separate exposures in different layers, or in the same layer in multiple exposures or stitching

Abstract

An image based overlay measurement is performed using an overlay target that includes shifted overlying gratings. The overlay target is imaged and an asymmetry is measured in the image of the overlaid gratings. The asymmetry is used to determine the overlay error. For each measurement direction, the overlay target may include two or more overlay measurement pads with different offsets between the top and bottom gratings. The measured asymmetries and offsets in the overlay measurement pads may be used to determine the overlay error, e.g., using self-calibration. The pitch and critical dimensions of the overlay target may be optimized to produce a greatest change of symmetry with overlay error for a numerical aperture and wavelength of light used by the image based metrology device.

Claims

1. A method of measuring an overlay error in a structure having an overlay target, the method comprising:

imaging the overlay target with an image based metrology device, the overlay target having a first top grating and a first bottom grating, wherein a nominal position of the first top grating with respect to the first bottom grating is the first top grating overlies the first bottom grating and is shifted with respect to the first bottom grating;

measuring an asymmetry in an image of the overlay target; and

using the asymmetry to determine the overlay error in the structure.

2. The method of claim 1, wherein in the nominal position a lateral separation between a first line in the first top grating and a nearest second line in the first bottom grating is less than five times an image resolution limit of the image based metrology device.

3. The method of claim 1, wherein the overlay target comprises a first overlay measurement pad and a second overlay measurement pad, the first overlay measurement pad having the first top grating and the first bottom grating, the second overlay measurement pad having a second top grating and a second bottom grating, wherein a nominal position of the overlay target is a center of the first top grating is shifted with respect to a center of the first bottom grating by a first offset and a center of the second top grating is shifted with respect to a center of the second bottom grating by a second offset that is different than the first offset.

4. The method of claim 3, wherein the asymmetry in the image is a first asymmetry in the first overlay measurement pad, and wherein using the asymmetry to determine the overlay error comprises:

measuring a second asymmetry in the second overlay measurement pad; and

using the first asymmetry, the first offset, the second asymmetry and second offset to determine the overlay error.

5. The method of claim 4, wherein using the first asymmetry, the first offset, the second asymmetry and second offset to determine the overlay error comprises:

producing a calibration of asymmetry response to overlay shift using the first asymmetry, the first offset, the second asymmetry and second offset; and

applying the calibration to at least one of the first asymmetry and the second asymmetry to determine the overlay error.

6. The method of claim 4, wherein using the first asymmetry, the first offset, the second asymmetry and second offset to determine the overlay error comprises:

self-calibrating using the first asymmetry, the first offset, the second asymmetry and second offset based on asymmetry response to overlay shift to calculate the overlay error.

7. The method of claim 4, wherein using the first asymmetry, the first offset, the second asymmetry and second offset to determine the overlay error comprises using a difference between the first asymmetry and the second asymmetry or a regression analysis with the first asymmetry and the second asymmetry.

8. The method of claim 7, wherein using the regression analysis with the first asymmetry and the second asymmetry comprises a linear regression analysis or a non-linear regression analysis using additional asymmetry values measured from additional overlay measurement pads in the overlay target.

9. The method of claim 3, wherein the overlay error is in a first measurement direction and is determined using a first asymmetry in the first overlay measurement pad and a second asymmetry in the second overlay measurement pad, the overlay target further comprising a third overlay measurement pad and a fourth overlay measurement pad and the method further comprises measuring a second overly error in a second measurement direction that is orthogonal to the first measurement direction using a third asymmetry in the third overlay measurement pad and a fourth asymmetry in the fourth overlay measurement pad.

10. The method of claim 3, wherein the first offset and the second offset are equal in magnitude but opposite in direction.

11. The method of claim 1, wherein the overlay error is in a first measurement direction and is determined using a first asymmetry of the first top grating and the first bottom grating in the first measurement direction, and the method further comprises measuring a second asymmetry of the first top grating and the first bottom grating in a second direction that is orthogonal to the first measurement direction and using the second asymmetry to determine a second overly error in the second measurement direction.

12. The method of claim 1, wherein using the asymmetry to determine the overlay error in the structure comprises converting the asymmetry to an overlay error using a calibration factor determined from a theoretical analysis.

13. The method of claim 1, wherein using the asymmetry to determine the overlay error in the structure comprises converting the asymmetry to an overlay error using a calibration factor determined from a different overlay target.

14. The method of claim 1, wherein the first top grating has a same pitch as the first bottom grating.

15. The method of claim 1, wherein the first top grating is on a same plane as the first bottom grating.

16. The method of claim 1, wherein the first top grating is on a different plane than the first bottom grating.

17. The method of claim 1, wherein a pitch and critical dimensions of the overlay target are optimized to produce a greatest change of symmetry with overlay error for a numerical aperture and wavelength of light used by the image based metrology device.

18. An image based metrology device comprising:

an illumination source to produce illumination to be incident on an overlay target on a sample, the overlay target having a first top grating and a first bottom grating, wherein a nominal position of the first top grating with respect to the first bottom grating is the first top grating overlies the first bottom grating and is shifted with respect to the first bottom grating;

an imaging system to image the overlay target; and

a processor coupled to receive the image of the overlay target from the imaging system, the processor configured to measure an asymmetry in the image of the overlay target, and use the asymmetry to determine the overlay error in the sample.

19. The image based metrology device of claim 18, wherein the imaging system has an image solution limit and wherein the overlay target in the nominal position has a lateral separation between a first line in the first top grating and a nearest second line in the first bottom grating that is less than five times the image resolution limit.

20. The image based metrology device of claim 18, wherein the overlay target comprises a first overlay measurement pad and a second overlay measurement pad, the first overlay measurement pad having the first top grating and the first bottom grating, the second overlay measurement pad having a second top grating and a second bottom grating, wherein a nominal position of the overlay target is a center of the first top grating is shifted with respect to a center of the first bottom grating by a first offset and a center of the second top grating is shifted with respect to a center of the second bottom grating by a second offset that is different than the first offset.

21. The image based metrology device of claim 20, wherein the asymmetry in the image is a first asymmetry in the first overlay measurement pad, and wherein the processor is configured to use the asymmetry to determine the overlay error by being configured to measure a second asymmetry in the second overlay measurement pad, use the first asymmetry and the second asymmetry to use the first asymmetry, the first offset, the second asymmetry and second offset to determine the overlay error.

22. The image based metrology device of claim 21, wherein the processor is configured to use the first asymmetry, the first offset, the second asymmetry and second offset to determine the overlay error by being configured to use a difference between the first asymmetry and the second asymmetry or a regression analysis with the first asymmetry and the second asymmetry.

23. The image based metrology device of claim 22, wherein the processor is configured to use the regression analysis with the first asymmetry and the second asymmetry by being configured to use a linear regression analysis or a non-linear regression analysis with additional asymmetry values measured from additional overlay measurement pads in the overlay target.

24. The image based metrology device of claim 20, wherein the overlay error is in a first measurement direction and is determined using a first asymmetry in the first overlay measurement pad and a second asymmetry in the second overlay measurement pad, the overlay target further comprising a third overlay measurement pad and a fourth overlay measurement pad and the processor is further configured to measure a second overly error in a second measurement direction that is orthogonal to the first measurement direction using a third asymmetry in the third overlay measurement pad and a fourth asymmetry in the fourth overlay measurement pad.

25. The image based metrology device of claim 20, wherein the first offset and the second offset are equal in magnitude but opposite in direction.

26. The image based metrology device of claim 18, wherein the overlay error is in a first measurement direction and is determined using a first asymmetry of the first top grating and the first bottom grating in the first measurement direction, and wherein the processor is further configured to measure a second asymmetry of the first top grating and the first bottom grating in a second measurement direction that is orthogonal to the first measurement direction and use the second asymmetry to determine a second overly error in the second measurement direction.

27. The image based metrology device of claim 18, wherein the processor is configured to use the asymmetry to determine the overlay error in the sample by being configured to convert the asymmetry to an overlay error using a calibration factor determined from a theoretical analysis.

28. The image based metrology device of claim 18, wherein the processor is configured to use the asymmetry to determine the overlay error in the sample by being configured to convert the asymmetry to an overlay error using a calibration factor determined from a different overlay target.

29. The image based metrology device of claim 18, wherein the first top grating has a same pitch as the first bottom grating.

30. The image based metrology device of claim 18, wherein the first top grating is on a same plane as the first bottom grating.

31. The image based metrology device of claim 18, wherein the first top grating is on a different plane than the first bottom grating.

32. The image based metrology device of claim 18, wherein a pitch and critical dimensions of the overlay target are optimized to produce a greatest change of symmetry with overlay error for a numerical aperture of the imaging system and wavelength of light produced by the illumination source.

33. A method of measuring an overlay error in a structure having an overlay target, the method comprising:

imaging the overlay target with an image based metrology device, the overlay target having a first overlay measurement pad having a first top grating that overlies a first bottom grating, wherein a center of the first top grating is nominally shifted with respect to a center of the first bottom grating by a first offset, the overlay target further having a second overlay measurement pad having a second top grating that overlies a second bottom grating, wherein a center of the second top grating is nominally shifted with respect to a center of the second bottom grating by a second offset that is different than the first offset;

measuring a first asymmetry in the first overlay measurement pad from an image of the overlay target;

measuring a second asymmetry in the second overlay measurement pad from the image of the overlay target; and

using the first asymmetry, the first offset, the second asymmetry, and the second offset to determine the overlay error in the structure.

34. The method of claim 33, wherein a lateral separation between a first line in the first top grating and a nearest second line in the first bottom grating is less than an image resolution limit of the image based metrology device causing proximity effects in the image of the overlay target.

35. The method of claim 33, wherein using the first asymmetry, the first offset, the second asymmetry, and the second offset to determine the overlay error in the structure comprises:

producing a calibration of asymmetry response to overlay shift using the first asymmetry, the first offset, the second asymmetry and second offset; and

applying the calibration to at least one of the first asymmetry and the second asymmetry to determine the overlay error.

36. The method of claim 33, wherein using the first asymmetry, the first offset, the second asymmetry and second offset to determine the overlay error comprises:

self-calibrating using the first asymmetry, the first offset, the second asymmetry and second offset based on asymmetry response to overlay shift to calculate the overlay error.

37. The method of claim 33, wherein using the first asymmetry, the first offset, the second asymmetry and second offset to determine the overlay error comprises using a difference between the first asymmetry and the second asymmetry or a regression analysis with the first asymmetry and the second asymmetry.

38. The method of claim 37, wherein using the regression analysis with the first asymmetry and the second asymmetry comprises a linear regression analysis or a non-linear regression analysis using additional asymmetry values measured from additional overlay measurement pads in the overlay target.

39. The method of claim 33, wherein the overlay error is determined for a first measurement direction, the overlay target further comprising a third overlay measurement pad and a fourth overlay measurement pad and the method further comprises measuring a second overly error in a second measurement direction that is orthogonal to the first measurement direction using a third asymmetry in the third overlay measurement pad and a fourth asymmetry in the fourth overlay measurement pad.