Method and apparatus for adaptive alignment

1. A method for aligning a wafer image with a reference image, comprising:

searching for a targeted reference position in a database for aligning the wafer image with the reference image; and

in response to a determination that the targeted reference position does not exist in the database:

identifying a current lock position and an area that encloses the current lock position on the wafer image;

computing an alignment score of the current lock position;

comparing the alignment score of the current lock position with stored alignment scores of positions previously selected in relation to aligning the wafer image with the reference image; and

aligning the wafer image with the reference image based on the comparing.

2. The method of claim 1 , wherein aligning the wafer image with the reference image based on the comparison further comprises:

in response to a determination that the alignment score of the current lock position satisfies a threshold condition:

storing location information and the alignment score of the current lock position at the database to facilitate a determination of a targeted reference position; and

aligning the area enclosing the current lock position with the reference image using the current lock position.

3. The method of claim 2 , wherein the alignment score of the current lock position satisfies the threshold condition when the alignment score of the current lock position is higher than the stored alignment scores of positions that are located within the area enclosing the current lock position, the positions that are located within the area enclosing the current lock position being the positions previously selected in relation to aligning the wafer image with the reference image.

4. The method of claim 1 , wherein aligning the wafer image with the reference image based on the comparison further comprises:

in response to a determination that the alignment score of the current lock position does not satisfy a threshold condition:

selecting a highest alignment score among the stored alignment scores; and

aligning the area enclosing the current lock position with the reference image using a lock position corresponding to the highest alignment score.

5. The method of claim 4 , wherein the threshold condition is not satisfied when the alignment score of the current lock position is lower than the stored alignment scores of positions that are located within the area enclosing the current lock position, the positions that are located within the area enclosing the current lock position being the positions previously selected in relation to aligning the wafer image with the reference image.

6. The method of claim 4 , wherein the lock position corresponding to the highest alignment score is a location that is in a field of view and that is different than the current lock position.

7. The method of claim 1 , wherein the area enclosing the current lock position is a field of view.

8. The method of claim 1 , wherein the area enclosing the current lock position is a portion of a field of view.

9. The method of claim 1 , wherein the alignment score of the current lock position and the stored alignment scores are determined based on a sum of squared differences in intensity:

SSD

=

∑

i

⁢

(

p

i

-

q

i

)

2

where p i and q i denote pixel intensity values of a wafer image and a reference image, respectively.

10. The method of claim 1 , wherein the alignment score of the current lock position and the stored alignment scores are determined based on correlation coefficients between two images:

r

=

∑

i

⁢

(

p

i

-

p

_

)

⁢

(

q

i

-

q

_

)

∑

i

⁢

(

p

i

-

p

_

)

2

⁢

∑

i

⁢

(

q

i

-

q

_

)

2

where p i and q i denote pixel intensity values of a wafer image and a reference image, respectively, and p and q denote mean intensity values of the wafer image and the reference image, respectively.

11. The method of claim 1 , wherein a size of the area enclosing the current lock position and a size of the area surrounding the targeted reference position is within millimeter to nanometer scale.

12. The method of claim 1 , wherein the current lock position is defined based on a comparison of numbers of features existing at different regions of a wafer corresponding to the wafer image.

13. The method of claim 1 , wherein the current lock position is defined based on a comparison of numbers of pattern edges existing at different regions of a wafer corresponding to the wafer image.

14. An electron beam inspection apparatus, comprising:

a controller having circuitry to cause the electron beam inspection apparatus to perform:

searching for a targeted reference position in a database; and

in response to a determination that the targeted reference position does not exist in the database:

defining a current lock position and an area enclosing the current lock position on the wafer image;

computing an alignment score of the current lock position;

comparing the alignment score of the current lock position with stored alignment scores of positions previously selected in relation to aligning the wafer image with the reference image; and

aligning the wafer image with a reference image based on the comparing.

15. A non-transitory computer readable medium storing a set of instructions that is executable by a controller of a device to cause the device to perform a method comprising:

searching for a targeted reference position in a database; and

in response to a determination that the targeted reference position does not exist in the database:

defining a current lock position and an area enclosing the current lock position on the wafer image;

computing an alignment score of the current lock position;

comparing the alignment score of the current lock position with stored alignment scores of positions previously selected in relation to aligning the wafer image with a reference image; and

aligning the wafer image with the reference image based on the comparing.