Structure and method for determining an overlay accuracy

1. An overlay target assembly adapted for use in determining an alignment accuracy of a first layer and a second layer of a multi-layered device, said assembly comprising:

at least two targets, each target having a first sub-structure of said first layer and a second sub-structure of said second layer that cooperate to indicate overlay error in a predetermined direction;

wherein, when said first layer and said second layer are correctly aligned:

said first sub-structure and said second sub-structure of at least one of said targets are offset with respect to each other by a programmed offset defined by a difference between a first center point of the first substructure and a second center point of the second substructure in the predetermined direction; and

said overlay target assembly is invariant to at least one transformation selected from rotation about a predetermined angle and reflection.

2. The overlay target assembly of claim 1 , wherein when, said first layer and said second layer are correctly aligned, said first sub-structure and said second sub-structure of each of said at least two targets are offset with respect to each other by a respective programmed offset.

3. The overlay target assembly of claim 1 , comprising at least four targets arranged in an n×n array.

4. The overlay target assembly of claim 3 , wherein said overlay target assembly is invariant to reflection about a mirror axis which extends in parallel to a row or a column of said n×n array.

5. The overlay target assembly of claim 4 , wherein:

said targets are arranged in quadrants of a Cartesian coordinate system spanned by an x axis and a y axis, wherein said x-axis corresponds to said mirror axis which extends in parallel to said row of said n x n array and said y-axis corresponds to said mirror axis which extends in parallel to said column of said n×n array;

said programmed offset of each target is defined by a respective offset vector; and

each offset vector is on a straight line running through an origin of said Cartesian coordinate system.

6. The overlay target assembly of claim 5 , wherein offset vectors of diametrical quadrants are point-symmetric with regard to the origin of the Cartesian coordinate systems.

7. The overlay target assembly of claim 5 , wherein said offset vectors are of said same length.

8. The overlay target assembly of claim 1 , wherein said overlay target assembly is invariant to a rotation about 90 degrees.

9. The overlay target assembly of claim 1 , wherein said overlay target assembly is invariant to a rotation about 180 degrees.

10. The overlay target assembly of claim 1 , wherein each target has a generally rectangular form.

11. The overlay target assembly of claim 1 , wherein each target has a dimension smaller than 4.5 μm.

12. The overlay target assembly of claim 1 , wherein said at least two targets are spaced equidistantly by a predetermined distance.

13. The overlay target assembly of claim 1 , wherein at least one of said targets is a target having a single symmetry axis.

14. The overlay target assembly of claim 1 , wherein of each target at least one of the first sub-structure and the second sub-structure has two or more symmetry axis.

15. The overlay target assembly of claim 1 , wherein two neighboring targets share a common target element.

16. The overlay target assembly of claim 1 , wherein said overlay target assembly is located on a semiconductor device.

17. The overlay target assembly of claim 16 , wherein said overlay target assembly is located in a chip area comprising active elements of said semiconductor device.

18. An overlay target structure adapted for use in determining an alignment accuracy of a first layer and a second layer of a multi-layered device, said structure comprising:

a first sub-structure of said first layer; and

a second sub-structure of said second layer, wherein:

the first and second sub-structures cooperate to indicate overlay error in a predetermined direction;

when said first layer and said second layer are correctly aligned, said first sub-structure and said second sub-structure are offset with respect to each other defined by a difference between a first center point of the first substructure and a second center point of the second substructure in the predetermined direction; and

said overlay target structure is invariant to at least one transformation selected from rotation about a predetermined angle and reflection.

19. A method, comprising:

forming an overlay target assembly comprising:

at least two targets, each target having a first sub-structure of a first layer and a second sub-structure of a second layer that cooperate to indicate overlay error in a predetermined direction;

wherein, when said first layer and said second layer are correctly aligned:

said first sub-structure and said second sub-structure of at least one of said targets are offset with respect to each other by a programmed offset defined by a difference between a first center point of the first substructure and a second center point of the second substructure in the predetermined direction; and

said overlay target assembly is invariant to at least one transformation selected from rotation about a predetermined angle and reflection; and

determining an alignment accuracy between said first layer and said second layer from a spatial relationship of said first sub-structure and said second sub-structure of said at least two targets.

20. The method of claim 19 , wherein said alignment accuracy between said first layer and said second layer is determined on the basis of an image asymmetry of each individual target.