Metrology method, patterning device, apparatus and computer program
A method of measuring overlay uses a plurality of asymmetry measurements from locations (LOI) on a pair of sub-targets ( 1032, 1034 ) formed on a substrate (W). For each sub-target, the plurality of asymmetry measurements are fitted to at least one expected relationship ( 1502, 1504 ) between asymmetry and overlay, based on a known bias variation deigned into the sub-targets. Continuous bias variation in one example is provided by varying the pitch of top and bottom gratings (P 1 /P 2 ). Bias variations between the sub-targets of the pair are equal and opposite (P 2 /P 1 ). Overlay (OV) is calculated based on a relative shift (xs) between the fitted relationships for the two sub-targets. The step of fitting asymmetry measurements to at least one expected relationship includes wholly or partially discounting measurements ( 1506, 1508, 1510 ) that deviate from the expected relationship and/or fall outside a particular segment of the fitted relationship.
1. A target formed on a substrate, the target comprising:
a first pair of sub-targets comprising:
a first sub-target; and
a second sub-target; and
a second pair of sub-targets comprising:
a third sub-target; and
a fourth sub-target,
wherein the first and second sub-targets are arranged adjacent to each other vertically and the third and fourth sub-targets are arranged on opposite sides of the first and second sub-targets, wherein left sides of the first and second sub-targets are adjacent to the third sub-target, wherein right sides of the first and second sub-targets are adjacent to the fourth sub-target,
wherein each sub-target comprises an overlay grating with a top layer and a bottom layer with different pitches, and
wherein the target comprises a continuous variation of bias values across each sub-target.
2. The target of claim 1 , wherein:
features in a top layer of the first sub-target are arranged with a first pitch;
features in a bottom layer of the first sub-target are arranged with a second pitch; and
the first pitch is larger than the second pitch.
3. The target of claim 1 , wherein:
features in a top layer of the second sub-target are arranged with a first pitch;
features in a bottom layer of the second sub-target are arranged with a second pitch; and
the first pitch is smaller than the second pitch.
4. The target of claim 1 , wherein the continuous variation of bias values across each sub-target is associated with an asymmetry-related property of the target.
5. The target of claim 1 , wherein the sub-targets in the first and second pairs have equal and opposite bias variations.
6. The target of claim 1 , wherein the first and second sub-targets have a same orientation that is perpendicular to an orientation of the third and fourth sub-targets.
7. The target of claim 1 , wherein the sub-targets of the first and second pairs are rectangular.
8. A patterning device for use in a lithographic apparatus, the patterning device comprising:
portions that define one or more device patterns; and
portions that define one or more metrology patterns,
wherein the one or more metrology patterns comprise a target, the target comprising:
a first pair of sub-targets comprising:
a first sub-target; and
a second sub-target; and
a second pair of sub-targets comprising:
a third sub-target; and
a fourth sub-target,
wherein the first and second sub-targets are arranged adjacent to each other vertically and the third and fourth sub-targets are arranged on opposite sides of the first and second sub-targets, wherein left sides of the first and second sub-targets are adjacent to the third sub-target, wherein right sides of the first and second sub-targets are adjacent to the fourth sub-target,
wherein each sub-target comprises an overlay grating with a top layer and a bottom layer with different pitches, and
wherein the target comprises a continuous variation of bias values across each sub-target.
9. The patterning device of claim 8 , wherein:
features in a top layer of the first sub-target are arranged with a first pitch;
features in a bottom layer of the first sub-target are arranged with a second pitch; and
the first pitch is larger than the second pitch.
10. The patterning device of claim 8 , wherein:
features in a top layer of the second sub-target are arranged with a first pitch;
features in a bottom layer of the second sub-target are arranged with a second pitch; and
the first pitch is smaller than the second pitch.
11. The patterning device of claim 8 , wherein the continuous variation of bias values across each sub-target is associated with an asymmetry-related property of the target.
12. The patterning device of claim 8 , wherein the sub-targets in the first and second pairs have equal and opposite bias variations.
13. The patterning device of claim 8 , wherein the first and second sub-targets have a same orientation that is perpendicular to an orientation of the third and fourth sub-targets.
14. The patterning device of claim 8 , wherein the sub-targets of the first and second pairs are rectangular.
15. A metrology apparatus comprising:
an illumination system configured to illuminate with radiation a target formed on a substrate; and
a detection system configured to detect scattered radiation arising from illumination of the target, the target comprising:
a first pair of sub-targets comprising:
a first sub-target; and
a second sub-target; and
a second pair of sub-targets comprising:
a third sub-target; and
a fourth sub-target,
wherein the first and second sub-targets are arranged adjacent to each other vertically and the third and fourth sub-targets are arranged on opposite sides of the first and second sub-targets, wherein left sides of the first and second sub-targets are adjacent to the third sub-target, wherein right sides of the first and second sub-targets are adjacent to the fourth sub-target,
wherein each sub-target comprises an overlay grating with a top layer and a bottom layer with different pitches, and
wherein the target comprises a continuous variation of bias values across each sub-target, and
wherein the metrology apparatus is configured to:
obtain a plurality of asymmetry measurements from locations on the target;
fit the plurality of asymmetry measurements to at least one expected relationship between asymmetry and a performance parameter, based on a bias variation between the locations on the target; and
derive a measure of the performance parameter from the fitting.
16. The metrology apparatus of claim 15 , wherein:
features in a top layer of the first sub-target are arranged with a first pitch;
features in a bottom layer of the first sub-target are arranged with a second pitch; and
the first pitch is larger than the second pitch.
17. The metrology apparatus of claim 15 , wherein:
features in a top layer of the second sub-target are arranged with a first pitch;
features in a bottom layer of the second sub-target are arranged with a second pitch; and
the first pitch is smaller than the second pitch.
18. The metrology apparatus of claim 15 , wherein the continuous variation of bias values across each sub-target is associated with an asymmetry-related property of the target.
19. The metrology apparatus of claim 15 , wherein the sub-targets in the first and second pairs have equal and opposite bias variations.
20. The metrology apparatus of claim 15 , wherein the first and second sub-targets have a same orientation that is perpendicular to an orientation of the third and fourth sub-targets.