Method of measuring a target, substrate, metrology apparatus, and lithographic apparatus
Disclosed is a method of measuring a target, an associated substrate, a metrology apparatus and a lithographic apparatus. In one arrangement the target comprises a layered structure. The layered structure has a first target structure in a first layer and a second target structure in a second layer. The method comprises illuminating the target with measurement radiation. Scattered radiation formed by interference between plural predetermined diffraction orders is detected. The predetermined diffraction orders are generated by diffraction of the measurement radiation from the first target structure and are subsequently diffracted from the second target structure. A characteristic of the lithographic process is calculated using the detected scattered radiation formed by the interference between the predetermined diffraction orders.
1. A method of measuring a target formed by a lithographic process, the target comprising a layered structure having a first target structure in a first layer and a second target structure in a second layer, the method comprising:
illuminating the target with measurement radiation;
detecting scattered radiation formed by interference between plural predetermined diffraction orders, wherein the predetermined diffraction orders are generated by double diffraction of the measurement radiation from the first target structure and the second target structure; and
calculating a characteristic of the lithographic process using the detected scattered radiation formed by the interference between the predetermined diffraction orders;
wherein the first target structure includes a two-dimensional periodic component,
wherein the second target structure includes a one-dimensional periodic component, and
wherein the one-dimensional periodic component overlaps the two-dimensional periodic component.
2. The method of claim 1 , wherein said characteristic of the lithographic process comprises an overlay error between the first target structure and the second target structure.
3. The method of claim 1 , wherein said predetermined diffraction orders comprise two equal and opposite diffraction orders.
4. The method of claim 1 , wherein:
the predetermined diffraction orders are generated by diffraction in reflection from the first target structure and the subsequent diffraction of the predetermined diffraction orders from the second target structure comprises diffraction in transmission through the second target structure; or
the predetermined diffraction orders are generated by diffraction in transmission through the first target structure and the subsequent diffraction of the predetermined diffraction orders from the second target structure comprises diffraction in reflection from the second target structure.
5. The method of claim 1 , wherein:
the target comprises three or more pairs of overlapping target sub-structures, each pair of overlapping target sub-structures comprising a first target sub-structure in the first target structure and a second target sub-structure in the second target structure;
each of the first target sub-structure and the second target sub-structure in each pair of overlapping target sub-structures comprises a first periodic component having the same pitch and orientation; and
each pair of overlapping target sub-structures is provided with a different overlay bias.
6. The method of claim 5 , wherein:
the detected scattered radiation formed by interference between the predetermined diffraction orders comprises a plurality of intensity sub-regions, each intensity sub-region having spatially uniform intensity and being formed by measurement radiation diffracted from a different respective pair of the three or more pairs of target sub-structures, and
the calculating of the characteristic of the lithographic process uses a level of intensity in each intensity sub-region to determine the characteristic of the lithographic process.
7. The method of claim 5 , wherein the predetermined diffraction orders are defined with respect to diffraction from the first periodic component in each pair of target sub-structures.
8. The method of claim 5 , wherein the overlay biases comprise one or more pairs of equal and opposite overlay biases.
9. The method of claim 5 , wherein the three or more pairs of target sub-structures comprises four pairs of target sub-structures.
10. The method of claim 9 , wherein the overlay biases comprise the following: −P8−d, P8+d, −P8+d and P8−d, where P is the pitch of the first periodic component and d is a predetermined constant.
11. The method of claim 5 , wherein either or both target sub-structures in each of the pairs of target sub-structures each comprises a second periodic component orientated in a different direction to the first periodic component.
12. The method of claim 11 , wherein the first periodic component is orientated perpendicularly to the second periodic component.
13. The method of claim 11 , wherein the target sub-structure with a second periodic component orientated in a different direction to the first periodic component comprises one or more of the following:
a checkerboard pattern formed from square elements or rectangular elements,
a tilted checkerboard pattern formed from square elements or rectangular elements rotated by a predetermined angle about an axis perpendicular to the plane of the checkerboard pattern, and
a two-dimensional grating.
14. The method of claim 1 , wherein the detected scattered radiation formed by interference between the predetermined diffraction orders comprises a fringe pattern.
15. The method of claim 14 , wherein the calculation of the characteristic of the lithographic process comprises calculating an overlay error between the first target structure and the second target structure by extracting a phase of the fringe pattern.
16. The method of claim 1 , wherein the predetermined diffraction orders are further generated by diffraction of the measurement radiation from the first target structure component and subsequent diffraction from the second first target structure.
17. The method of claim 1 , wherein the predetermined diffraction orders are generated by diffraction of the measurement radiation from the second target structure and are subsequently diffracted from the first target structure.
18. A substrate comprising:
a target formed by a lithographic process, the target comprising:
a layered structure having a first target structure in a first layer and a second target structure in a second layer,
wherein the first target structure includes a two-dimensional periodic component,
wherein the second target structure includes a one-dimensional periodic component, and
wherein the one-dimensional periodic component overlaps the two-dimensional periodic component,
wherein the first target structure and the second target structure are configured to allow detection of radiation scattered from the target when the target is illuminated with measurement radiation, the detected scattered radiation being formed by interference between plural predetermined diffraction orders,
wherein the predetermined diffraction orders are generated by double diffraction of the measurement radiation from the first target structure and the second target structure.
19. The substrate of claim 18 , wherein the target is an overlay target for measurement of overlay error, the overlay target being configured such that the detected scattered radiation formed by interference between the predetermined diffraction orders varies as a function of overlay error between the first target structure and the second target structure.
20. A lithographic apparatus configured to produce the substrate of claim 18 by forming the target on the substrate.
21. A metrology apparatus comprising:
an illumination system configured to illuminate with measurement radiation a target produced using a lithographic process on a substrate; and
a detection system configured to detect scattered radiation arising from illumination of the target, wherein:
the metrology apparatus is operable to perform a method of measuring the target formed by the lithographic process, the target comprising a layered structure having a first target structure in a first layer and a second target structure in a second layer, the method comprising:
illuminating the target with the measurement radiation;
detecting scattered radiation formed by interference between plural predetermined diffraction orders, wherein the predetermined diffraction orders are generated by double diffraction of the measurement radiation from the first target structure and the second target structure; and
calculating a characteristic of the lithographic process using the detected scattered radiation formed by the interference between the predetermined diffraction orders;
wherein the first target structure includes a two-dimensional periodic component,
wherein the second target structure includes a one-dimensional periodic component, and
wherein the one-dimensional periodic component overlaps the two-dimensional periodic component.