Overlay mark and method of measuring the same

1. An overlay mark, comprising:

a first feature extending only along a first longitudinal direction;

a second feature extending along a second longitudinal direction different from the first longitudinal direction; and

a third feature extending along the first longitudinal direction and the second longitudinal direction, the third feature being continuous between the first longitudinal direction and the second longitudinal direction;

wherein the first feature is in a first layer, the second feature is in a second layer over the first layer, and the third feature is in a third layer over the second layer, and the third feature is an opening in the third layer defining a mask.

2. The overlay mark of claim 1 , wherein the first longitudinal direction is substantially perpendicular to the second longitudinal direction.

3. The overlay mark of claim 1 , wherein the first feature comprises a plurality of alignment segments substantially parallel to each other.

4. The overlay mark of claim 1 , wherein the second feature comprises a plurality of alignment segments substantially parallel to each other.

5. The overlay mark of claim 1 , wherein the third feature comprises a plurality of alignment segments substantially parallel to each other extending along the first longitudinal direction and a plurality of alignment segments substantially parallel to each other extending along the second longitudinal direction.

6. The overlay mark of claim 1 , wherein the third feature comprises four alignment segments forming a rectangular pattern or a square pattern.

7. The overlay mark of claim 1 , wherein the first feature comprises an isolation structure.

8. The overlay mark of claim 1 , wherein the second feature comprises a gate electrode.

9. A device, comprising:

a substrate; and

an overlay mark over the substrate, wherein the overlay mark comprises:

a first feature extending only along a first longitudinal direction;

a second feature extending along a second longitudinal direction different from the first longitudinal direction; and

a third feature extending along both the first longitudinal direction and the second longitudinal direction, the third feature being continuous between the first longitudinal direction and the second longitudinal direction;

wherein the first feature is in a first layer, the second feature is in a second layer over the first layer, and the third feature is in a third layer over the second layer, and the third feature is an opening in the third layer defining a mask.

10. The device of claim 9 , wherein the first longitudinal direction is substantially perpendicular to the second longitudinal direction.

11. The device of claim 9 , wherein the first feature comprises a plurality of alignment segments substantially parallel to each other.

12. The device of claim 9 , wherein the second feature comprises a plurality of alignment segments substantially parallel to each other.

13. The device of claim 9 , wherein the third feature comprises a plurality of alignment segments substantially parallel to each other extending along the first longitudinal direction and a plurality of alignment segments substantially parallel to each other extending along the second longitudinal direction.

14. A method of adjusting multi-layer overlay alignment, comprising:

providing a first feature in a first layer over a substrate, wherein the first feature comprises a plurality of first alignment segments substantially parallel to each other extending only along an X direction;

providing a second feature in a second layer over the first layer, wherein the second feature comprises a plurality of second alignment segments substantially parallel to each other extending along a Y direction different from the X direction;

providing a third feature in a third layer over the second layer, wherein the third feature comprises a plurality of third alignment segments substantially parallel to each other extending along the X direction and a plurality of fourth alignment segments substantially parallel to each other extending along the Y direction, the third feature being continuous between the X direction and the Y direction;

measuring an X-directional deviation between the fourth alignment segments and the second alignment segments;

calculating an X-directional offset value by the X-directional deviation;

measuring a Y-directional deviation between the third alignment segments and the first alignment segments;

calculating a Y-directional offset value by the Y-directional deviation; and

using the X-directional offset value or the Y-directional offset value to compensate for an overlay error,

wherein the third feature is an opening in the third layer defining a mask.

15. The method of claim 14 , wherein the step of measuring an X-directional deviation between the fourth alignment segments and the second alignment segments comprises:

defining a centerline X 1 parallel to the fourth alignment segments;

defining a centerline X 2 parallel to the second alignment segments; and

calculating the X-directional deviation between the centerline X 1 and the centerline X 2 .

16. The method of claim 14 , wherein the step of measuring a Y-directional deviation between the third alignment segments and the first alignment segments comprises:

defining a centerline Y 1 parallel to the third alignment segments;

defining a centerline Y 2 parallel to the first alignment segments; and

calculating the Y-directional deviation between the centerline Y 1 and the centerline Y 2 .

17. The method of claim 14 , wherein the step of measuring an X-directional deviation between the fourth alignment segments and the second alignment segments comprises:

detecting an intensity I 1 of a positive first order diffraction on gratings of the fourth alignment segments and the second alignment segments;

detecting an intensity I 2 of a negative first order diffraction on gratings of the fourth alignment segments and the second alignment segments; and

calculating an intensity difference of I 1 and I 2 on gratings of the fourth alignment segments and the second alignment segments, wherein the X-directional deviation is proportional to the intensity difference.

18. The method of claim 14 , wherein the step of measuring a Y-directional deviation between the third alignment segments and the first alignment segments comprises:

detecting an intensity I 3 of a positive first order diffraction on gratings of the third alignment segments and the first alignment segments;

detecting an intensity I 4 of a negative first order diffraction on gratings of the third alignment segments and the first alignment segments; and

calculating an intensity difference of I 3 and I 4 on gratings of the third alignment segments and the first alignment segments, wherein the Y-directional deviation is proportional to the intensity difference.

19. The method of claim 14 , further comprising repeating a photolithography process if the overlay error is larger than an acceptable deviation.