Apparatus and methods for determining overlay of structures having rotational or mirror symmetry
Disclosed are overlay targets having flexible symmetry characteristics and metrology techniques for measuring the overlay error between two or more successive layers of such targets. Techniques for imaging targets with flexible symmetry characteristics and analyzing the acquired images to determine overlay or alignment error are disclosed.
1. A semiconductor target for determining a relative shift between two or more successive layers of a substrate, the target comprising:
a plurality of first structures formed in a first layer, and the first structures having a first center of symmetry (COS), the first structures being aperiodic; and
a plurality of second structures formed in a second layer, and the second structures having a second COS, the second structures being aperiodic,
wherein the difference between the first COS and the second COS corresponds to an overlay error between the first and second layer and wherein the first and second structures have a 180° rotational symmetry, without having a 90° rotational symmetry, with respect to the first and second COS, respectively.
2. The target of claim 1 , wherein the first and second structures are in the form of device structures.
3. The target of claim 1 , wherein the first structures include a first set of sub-structures that each has a first shape and a second set of sub-structures that each has a second shape that differs from the first shape.
4. The target of claim 3 , wherein the second structures include a third set of sub-structures that each has a third shape and a fourth set of sub-structures that each has a fourth shape that differs from the third shape.
5. The target of claim 4 , wherein the first, second, third, and fourth shapes differ from each other.
6. The target of claim 1 , wherein the first and second structures are image-based overlay targets.
7. The target of claim 1 , wherein a difference between the first COS and the second COS that is greater than a known offset between the first and second COS corresponds to an overlay error between the first and second layer.
8. The target of claim 1 , further comprising an opaque layer deposited over the first or second structures.
9. The target of claim 1 , wherein the first or second layer is a dummy layer.
10. A wafer having the target of claim 1 and a plurality of dies, wherein the target is formed in a scribe line located between at least some of the dies.
11. A wafer having the target of claim 1 and a plurality of dies, wherein the target is formed within a one of the dies.