Lithographic apparatus, substrate and device manufacturing method
A method uses a lithographic apparatus to form an inspection target structure upon a substrate. The method comprises forming the periphery of the inspection target structure so as to provide a progressive optical contrast transition between the inspection target structure and its surrounding environment. This may be achieved by providing a progressive change in the optical index at the periphery of the target structure.
1. A method comprising:
illuminating a target structure using a spot size that is larger than the target structure, wherein the target structure includes a progressive optical contrast transition at peripheral edges of the target structure; and
measuring a diffraction pattern of the target structure, wherein a diffraction effect at the peripheral edges of the target structure is substantially reduced by the progressive optical contrast transition.
2. The method of claim 1 , wherein the progressive optical contrast transition provides a progressive change in the optical index at the peripheral edges of the target structure.
3. The method of claim 2 , wherein the progressive optical contrast transition includes an outermost element along one or more of the target structure's peripheral edges, the height of the element decreasing towards the peripheral edge.
4. The method of claim 3 , wherein the outermost element is wider than the other elements of which the target structure is comprised.
5. The method of claim 2 , wherein:
the progressive optical contrast transition includes peripheral elements along one or more of the target structure's peripheral edges; and
along any one peripheral edge, the optical index of each peripheral element differs progressively toward the peripheral edge.
6. The method of claim 5 , wherein the peripheral elements are lines wherein, along any one peripheral edge, the line width of each line decreases progressively toward the peripheral edge.
7. The method of claim 5 , wherein:
the peripheral elements are broken lines; and
along any one peripheral edge, the ratio of line element to line break for each broken line decreases progressively toward the peripheral edge.
8. The method of claim 1 , wherein the progressive optical contrast transition is formed by altering the patterning device using Optical Proximity Correction techniques.
9. The method of claim 1 , wherein the target structure is smaller than about 12 μm along any edge of the target structure.
10. The method of claim 1 , wherein the target structure is a repetitive structure usable by an inspection apparatus for measuring a parameter of the lithographic process.
11. The method of claim 1 , further comprising:
using an inspection apparatus to perform a measurement of an aspect of the target structure and to determine a parameter of a lithographic process from the measurement.
12. A substrate comprising:
a target structure formed thereon by a lithographic process, the target structure including a progressive optical contrast transition at peripheral edges of the target structure,
wherein the progressive optical contrast transition is configured to reduce a diffraction effect at the peripheral edges of the target structure when the target structure is illuminated with a spot size that is larger than the target structure.
13. The substrate of claim 12 , wherein the progressive optical contrast transition provides a progressive change in the optical index at the peripheral edges of the target structure.
14. The substrate of claim 13 , wherein the progressive optical contrast transition includes an outermost element along one or more of the target structure's peripheral edges, the height of the element decreasing towards the peripheral edge.
15. The substrate of claim 14 , wherein the outermost element is wider than the other elements of which the target structure is comprised.
16. The substrate of claim 13 , wherein:
the progressive optical contrast transition includes peripheral elements along one or more of the target structure's peripheral edges; and
along any one peripheral edge, the optical index of each peripheral element differs progressively toward the peripheral edge.
17. The substrate of claim 16 , wherein:
the peripheral elements are lines; and
along any one peripheral edge, the line width of each line decreases progressively toward the peripheral edge.
18. The substrate of claim 16 , wherein:
the peripheral elements are broken lines; and
along any one peripheral edge, the ratio of line element to line break for each broken line decreases progressively toward the peripheral edge.
19. The substrate of claim 12 , wherein the target structure is smaller than 12 μm along any edge of the target structure.
20. The substrate of claim 12 , wherein the target structure is a repetitive structure usable by an inspection apparatus for measuring a parameter of the lithographic process.
21. A non-transitory computer-readable storage device having instructions stored thereon, execution of which, by a computing device, causes the computing device to control an inspection apparatus to perform operations comprising:
illuminating a target structure using a spot size that is larger than the target structure, wherein the target structure includes a progressive optical contrast transition at peripheral edges of the target structure; and
measuring a diffraction pattern of the target structure, wherein a diffraction effect at the peripheral edges of the target structure is substantially reduced by the progressive optical contrast transition.
22. An inspection apparatus, comprising:
an optical system configured to direct a radiation beam onto a target structure patterned on a wafer, wherein the target structure includes a progressive optical contrast transition at peripheral edges of the target structure; and
one or more sensors configured to receive light from the target structure,
wherein a spot size of the radiation beam is larger than the target structure, and
wherein a diffraction effect at the peripheral edges of the target structure is substantially reduced by the progressive optical contrast transition.
23. The inspection apparatus of claim 22 , wherein the progressive optical contrast transition includes a progressive change in the optical index at the peripheral edges of the target structure.
24. The inspection apparatus of claim 23 , wherein the progressive optical contrast transition includes an outermost element along one or more of the target structure's peripheral edges, the height of the element decreasing towards the peripheral edge.
25. The inspection apparatus of claim 24 , wherein the outermost element is made wider than the other elements of which the target structure is comprised.
26. The inspection apparatus of claim 23 , wherein:
the progressive optical contrast transition includes peripheral elements along one or more of the target structure's peripheral edges; and
along any one peripheral edge, the optical index of each peripheral element differs progressively toward the peripheral edge.
27. The inspection apparatus of claim 26 , wherein:
the peripheral elements are lines; and
along any one peripheral edge, the line width of each line decreases progressively toward the peripheral edge.
28. The inspection apparatus of claim 26 , wherein:
the peripheral elements are broken lines; and
along any one peripheral edge, the ratio of line element to line break for each broken line decreases progressively toward the peripheral edge.
29. The inspection apparatus of claim 22 , wherein the target structure is smaller than about 12 μm along any edge.