Fault discrimination and calibration of scatterometry overlay targets

1. A scatterometry overlay target comprising at least three cells associated with a measurement direction, wherein two of the at least three cells comprise periodic structures at different target layers, having a same pitch and opposite offsets between the two cells, wherein at least a third cell of the at least three cells comprises a periodic structure with the same pitch at only one of the target layers, and wherein the periodic structure of at least the third cell is continuous across three cells; wherein at least the third cell adjoins at least one of the two cells.

2. The target of claim 1 , wherein at least the third cell comprises a single cell positioned adjacent to the two cells with periodic structures at different target layers and opposite offsets.

3. The target of claim 1 , wherein at least the third cell comprises at least one cell positioned adjacent to the two cells with periodic structures at different target layers and opposite offsets, wherein the at least one cell is positioned to separate, spatially, the target from other wafer structures.

4. The target of claim 1 , wherein at least the third cell comprises at least one cell positioned adjacent to the two cells with periodic structures at different target layers and opposite offsets, wherein the at least one cell is positioned on an edge of a pad holding the two cells.

5. The target of claim 1 , wherein at least the third cell is located on a different pad than the two cells with periodic structures at different target layers and opposite offsets.

6. The target of claim 1 , wherein at least the third cell comprises at least two cells, and wherein the at least two cells have the periodic structures at the different target layers.

7. The target of claim 6 , wherein the different target layers comprise a previous layer and a current layer and wherein one of the at least two cells comprises the periodic structure at the previous layer and another one of the at least two cells comprises the periodic structure at the current layer.

8. The target of claim 1 , wherein at least the third cell comprises a single cell positioned between the two cells with periodic structures at different target layers and opposite offsets.

9. The target of claim 8 , wherein the different target layers comprise a previous layer and a current layer and wherein the single cell comprises the periodic structure at the previous layer.

10. The target of claim 8 , wherein the different target layers comprise a previous layer and a current layer and wherein the single cell comprises the periodic structure at the current layer.

11. A method comprising incorporating in a scatterometry overlay target comprising two cells with periodic structures at different target layers, having a same pitch and opposite offsets between the two cells, at least a third cell comprising a periodic structure with the same pitch at only one of the target layers, wherein the periodic structure of at least the third cell is continuous across three cells and at least the third cell adjoins at least one of the two cells.

12. The method of claim 11 , wherein at least the third cell comprises a single cell positioned between and/or adjacent to the two cells.

13. The method of claim 11 , wherein at least the third cell comprises at least two cells having the periodic structures in at least a previous and a current layers.

14. The method of claim 11 , further comprising deriving an estimation of process-related inaccuracy from scatterometry measurements of at least the third cell and correcting a scatterometry overlay measurement based on the two cells by the process-related inaccuracy estimation.

15. The method of claim 11 , further comprising deriving an estimation of illumination-related inaccuracy from pupil image measurements of at least the third cell and correcting a scatterometry overlay measurement based on the two cells by the illumination-related inaccuracy estimation.

16. The method of claim 11 , further comprising estimating, from scatterometry measurements of at least the third cell, target health and indicating faulty process steps which reduce the estimated target health.

17. The method of claim 11 , further comprising correcting overlay measurements of the two cells in the target, using scatterometry measurements of at least the third cell.

18. The method of claim 11 , further comprising calibrating metrology simulations through intermediate measurement results derived from scatterometry measurements of at least the third cell.

19. The method of claim 11 , further comprising detecting, using scatterometry measurements of at least the third cell, the periodic structure with which a parasitic grating is associated, and correcting overlay measurements of the two cells in the target, accordingly.

20. The method of claim 11 , further comprising separating illumination asymmetry and periodic structure asymmetry effects by carrying out scatterometry measurements of at least the third cell in at least two orientations.

21. The method of claim 11 , further comprising deriving a reference image from pupil images of at least the third cell.

22. The method of claim 21 , wherein the derivation of the reference image from the pupil images is carried out inline.

23. The method of claim 11 , further comprising carrying out scatterometry measurements of at least the third cell, having the periodic structure in a previous layer, before complete production of the target.

24. The method of claim 23 , further comprising carrying out multiple temporal measurements of target structures of at least the third cell, as they are being processed, to provide realtime feedback over production quality.

25. A computer program product comprising a non-transitory computer readable storage medium having computer readable program embodied therewith, the computer readable program configured to carry out the method of claim 11 .

26. A metrology module comprising the computer program product of claim 25 .