Single cell scatterometry overlay targets
Scatterometry overlay (SCOL) single cell targets are provided, along with target design methods and measurement methods which employ the single cell SCOL targets for in-die metrology measurements, utilizing the small size of the target along with maintained optical performance due to the design of the target. Disclosed single cell targets comprise a lattice of elements at two or more layers which is periodic two or more measurement directions. Elements in different layers are offset with respect to each other and may have the same pitch along the measurement directions. Measurement algorithms are also provided to derive metrology measurements such as overlays from the single cell targets, possibly simultaneously in both (or more) measurement directions, reducing measurement time and enhancing the metrology throughput. Positioning the small targets in-die provides more accurate metrology results which are less sensitive to process variation.
1. A scatterometry overlay (SCOL) target comprising a single cell with a lattice of elements including at least at two layers wherein:
the lattice is periodic along at least two orthogonal measurement directions and a single diagonal direction,
the elements in one of the layers are offset with respect to the elements in another one of the layers along the at least two orthogonal measurement directions across the lattice of elements, and
each element in one of the layers at least partially overlaps a corresponding element in another one of the layers in the single diagonal direction across the lattice of elements,
wherein an overlay estimation for the SCOL target is derivable from measurements of differential signals in the at least two orthogonal measurement directions and the single diagonal direction.
2. The SCOL target of claim 1 , wherein the elements are arranged with corresponding pitches and offsets.
3. The SCOL target of claim 1 , wherein one or more pitches of the elements along the at least two orthogonal measurement directions are equal.
4. The SCOL target of claim 1 , wherein the elements are rectangular.
5. The SCOL target of claim 1 , wherein the elements are 10 μm or smaller.
6. A wafer comprising:
a plurality of SCOL targets, wherein at least some of the SCOL targets comprise a single cell with a lattice of elements including at least at two layers wherein:
the lattice is periodic along at least two orthogonal measurement directions and a single diagonal direction,
the elements in one of the layers are offset with respect to the elements in another one of the layers along the at least two orthogonal measurement directions across the lattice of elements, and
each element in one of the layers at least partially overlaps a corresponding element in another one of the layers in the single diagonal direction across the lattice of elements,
wherein an overlay estimation for each of the at least some of the SCOL targets is derivable from measurements of differential signals in the at least two orthogonal measurement directions and the single diagonal direction of each of the at least some of the SCOL targets.
7. A metrology measurement method comprising:
measuring differential signals from a scatterometry overlay (SCOL) target, wherein the SCOL target comprises a single cell with a lattice of elements including at least at two layers wherein:
the lattice is periodic along at least two orthogonal measurement directions and a single diagonal direction,
the elements in one of the layers are offset with respect to the elements in another one of the layers along the at least two orthogonal measurement directions across the lattice of elements, wherein the differential signals are measured simultaneously along the at least two orthogonal measurement directions, and
each element in one of the layers at least partially overlaps a corresponding element in another one of the layers in the single diagonal direction across the lattice of elements,
wherein the differential signals are measured in the at least two orthogonal measurement directions and the single diagonal direction; and
deriving one or more overlay estimations along the at least two orthogonal measurement directions and the single diagonal direction from the measured differential signals.
8. The method of claim 7 , wherein the overlay estimations are derived according to:
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where ε x and ε y denote overlays along the measurement directions, D x , D y , D xy , D xy denote measured differential signals along respective directions and f 0 denotes the offset between the elements at the two layers.
9. The method of claim 7 , wherein the at least two orthogonal measurement directions comprise at least three lattice vectors.
10. The method of claim 7 , wherein the elements are arranged with corresponding pitches and offsets.
11. The method of claim 7 , wherein one or more pitches of the elements along the at least two orthogonal measurement directions are equal.
12. The method of claim 7 , wherein the differential signals are simultaneously measured in the at least two orthogonal measurement directions and the single diagonal direction.
13. The method of claim 7 , wherein the elements are 10 μm or smaller.
14. A system comprising:
a controller, the controller including one or more processors and memory, the memory storing program instructions configured to cause the one or more processors to:
measure differential signals from a scatterometry overlay (SCOL) target, wherein the SCOL target comprises a single cell with a lattice of elements including at least at two layers wherein:
the lattice is periodic along at least two orthogonal measurement directions and a single diagonal direction,
the elements in one of the layers are offset with respect to the elements in another one of the layers along the at least two orthogonal measurement directions across the lattice of elements, wherein the differential signals are measured simultaneously along the at least two orthogonal measurement directions, and
each element in one of the layers at least partially overlaps a corresponding element in another one of the layers in the single diagonal direction across the lattice of elements,
wherein the differential signals are measured in the at least two orthogonal measurement directions and the single diagonal direction; and
derive one or more overlay estimations along the at least two orthogonal measurement directions and the single diagonal direction from the measured differential signals.
15. The system of claim 14 , further comprising a metrology sub-system including one or more sensors, wherein the controller is communicatively coupled to the one or more sensors of the metrology sub-system.
16. The system of claim 14 , wherein the at least two orthogonal measurement directions comprise at least three lattice vectors.
17. The system of claim 14 , wherein the elements are arranged with corresponding pitches and offsets.
18. The system of claim 14 , wherein one or more pitches of the elements along the at least two orthogonal measurement directions are equal.
19. The system of claim 14 , wherein the differential signals are simultaneously measured in the at least two orthogonal measurement directions and the single diagonal direction.
20. The system of claim 14 , wherein the elements are 10 μm or smaller.