Continuously varying offset mark and methods of determining overlay

• US 20050195398A1
• Filed: 02/16/2005
• Published: 09/08/2005
• Est. Priority Date: 12/05/2002
• Status: Active Grant

• Current Assignee: Kla-Tencor Technologies Corporation (KLA Corporation)
• Original Assignee: Kla-Tencor Technologies Corporation (KLA Corporation)
• Inventors: Seligson, Joel L., Adel, Michael E., Kandel, Daniel
• Granted Patent: US 7,440,105 B2
• Time in Patent Office: Days
• US Class Current: 356/401
• CPC Class Codes: B82Y 10/00 Nanotechnology for information processing, storage or transmission, e.g. quantum computing or single electron logic
• CPC Class Codes: B82Y 40/00 Manufacture or treatment of nanostructures
• CPC Class Codes: G03F 7/0002 Lithographic processes using patterning methods other than those involving the exposure to radiation, e.g. by stamping
• 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
• CPC Class Codes: G03F 7/70683 Mark designs
• CPC Class Codes: H01L 22/12 for structural parameters, e.g. thickness, line width, refractive index, temperature, warp, bond strength, defects, optical inspection, electrical measurement of structural dimensions, metallurgic measurement of diffusions electrical measurement of diffusions H01L22/14

Abstract

The present invention relates to overlay marks and methods for determining overlay error. One aspect of the present invention relates to a continuously varying offset mark. The continuously varying offset mark is a single mark that includes over laid periodic structures, which have offsets that vary as a function of position. By way of example, the periodic structures may correspond to gratings with different values of a grating characteristic such as pitch. Another aspect of the present invention relates to methods for determining overlay error from the continuously varying offset mark. The method generally includes determining the center of symmetry of the continuously varying offset mark and comparing it to the geometric center of the mark. If there is zero overlay, the center of symmetry tends to coincide with the geometric center of the mark. If overlay is non zero (e.g., misalignment between two layers), the center of symmetry is displaced from the geometric center of the mark. The displacement in conjunction with the preset gain of the continuously varying offset mark is used to calculate the overlay error.

Claims

1. A method of extracting overlay information from an overlay target having over laid first and second periodic structures designed with a preset gain factor, said method comprising:

obtaining a spatial array of optical data along the overlay target;

determining the center of symmetry of the overlay target based on the spatial array of optical data;

calculating a distance X between the center of symmetry and the geometric center of the overlay target; and

determining the overlay error by dividing the distance X by the preset gain factor.

2. The method as recited in claim 1 wherein the first periodic structure has a first value of an attribute of the periodic structure, the second periodic structure has a second value of an attribute of the periodic structure, and there is a small difference between the first and second values, and wherein the gain factor is determined by dividing the first value by the difference between the first and second values.

3. The method as recited in claim 2 wherein the difference between the first and second values is about 1.0%.

4. The method as recited in claim 2 wherein the attribute is selected from pitch or rotation.

5. The method as recited in claim 1 wherein the optical data is collected in a single optical measurement.

6. The method as recited in claim 1 wherein the optical data is collected via a plurality of optical measurements.

7. The method as recited in claim 1 wherein the center of symmetry of the overlay target is determined using a self correlation algorithm.

8. An overlay target comprising:

a first periodic structure located on a first layer, the first periodic structure having a first value of an attribute of a periodic structure; and

a second periodic structure located on a second layer, and positioned over the first periodic structure, the second periodic structure having at least a second value of the attribute of a periodic structure, the first and second values having a substantially small difference.

9. The overlay target as recited in claim 8 wherein the attribute is pitch.

10. The overlay target as recited in claim 8 wherein the attribute is rotation.

11. The overlay target as recited in claim 8 wherein the second value is larger than the first value.

12. The overlay target as recited in claim 8 wherein the second value is smaller than the first value.

13. The overlay target as recited in claim 8 wherein the periodic structures are one dimensional periodic structures.

14. The overlay target as recited in claim 13 wherein the first and second periodic structures are gratings oriented in the same direction.

15. The overlay target as recited in claim 8 wherein the periodic structures are two dimensional periodic structures.

16. The overlay target as recited in claim 15 wherein the periodic structures each consist of a plurality of sub segments oriented in two directions.

17. The overlay target as recited in claim 8 wherein the difference between the first and second values is less than about 10%.

18. The overlay target as recited in claim 8 wherein the difference between the first and second values is less than about 5%.

19. The overlay target as recited in claim 8 wherein the difference between the first and second values is less than or equal to about 1.0%.

20. The overlay target as recited in claim 8 wherein the difference between the first and second values is between about 0.01% and about 0.99%.

21. The overlay target as recited in claim 20 wherein the difference between the first and second values is between about 0.10% and about 0.99%.

22. An overlay target comprising:

a first grating positioned in a first layer and having a first pitch; and

a second grating positioned in a second layer and having a second pitch, the first and second pitches having a substantially small difference.

23. The overlay target as recited in claim 22 wherein the difference between the first pitch and second pitch is about 1.0% of one another.

24. An overlay target comprising:

a first portion located on a first layer, the first portion including a first grating having a first value of a grating characteristic;

a second portion located on a second layer and centered above the first portion, the first and second portions having substantially the same dimensional profile, the second portion including a second grating and third grating that are positioned side by side, the second grating having a second value of a grating characteristic, and the third grating having a third value of a grating characteristic, the second value being slightly larger than the first value, the third value being slightly smaller than the first value.

25. The overlay target as recited in claim 24 wherein the difference between the first value and the second and third values is about 1.0%.

26. A method for determining overlay error, comprising:

providing an overlay target comprising over laid periodic structures, the periodic structures having a different value for an attribute of the periodic structures, the different values for the attribute of the periodic structures having a substantially small difference;

obtaining an array of spectra across the overlay target;

finding the null point of the array of spectra, the null point being the point that produces a difference spectra that is substantially zero over the entire spectral range, the null point indicating the center of symmetry of the overlay target;

calculating a distance X between the center of symmetry and the geometric center of the overlay target;

calculating a ratio D between one of the values or an average of the values and the difference between the first and second values; and

determining the overlay error by dividing the distance X by the ratio D.