Calibration of lithographic apparatus
System parameters are checked through self-assessment of a production wafer without a reference or a monitor wafer. In particular, exposure errors and substrate table positioning errors can be corrected for.
1. A method, comprising:
detecting errors in system parameters of a lithographic apparatus, wherein the detecting errors comprises:
performing a plurality of exposures on a substrate, each exposure being carried out on an exposure area on the substrate and forming one or more markers within the exposure area, wherein neighboring exposure areas share at least one overlapping region; and
measuring the overlay between the markers formed from the neighboring exposures in the overlapping regions and measuring the positions of the markers across the substrate;
combining differences in the measured positions of the markers between exposures in the overlapping regions with the measured positions of the other markers to obtain absolute values of system parameter errors; and
deriving calibration correction factors from the obtained absolute values of system parameter errors.
2. The method of claim 1 , wherein the plurality of exposures form one or more first marker portions in one region of the exposure area and one or more second marker portions in another region of the exposure area; and wherein the overlapping regions are such that the first marker portions from one exposure of the plurality of exposures are designed to combine with the second marker portions from a neighboring exposure area to form a composite marker.
3. The method of claim 1 , wherein the substrate is a production wafer.
4. The method of claim 1 , wherein the substrate comprises a pre-etched reference layer.
5. The method of claim 1 , wherein the detecting errors comprises measuring the overlay between the markers of neighboring exposure areas that are scanned in different directions.
6. The method of claim 5 , wherein the different directions are opposite to each other.
7. The method of claim 1 , wherein the detecting errors comprises measuring the overlay between the markers of neighboring exposure areas that are stepped in different directions.
8. The method of claim 7 , wherein the different directions are opposite to each other.
9. The method of claim 1 , wherein the detecting errors comprises calculating a set of coefficients used in a polynomial representation of the system parameters based on the difference in position of the markers between exposures in the overlapping regions.
10. The method of claim 1 , wherein the system parameter errors comprise scanning errors.
11. The method claim 1 , wherein the system parameter errors comprise stepping errors.
12. The method of claim 1 , wherein the system parameter errors comprise substrate table positioning errors.
13. The method of claim 12 , wherein the substrate table positioning errors comprise information relating to irregularities in a mirrored side-wall surface of a substrate table.
14. A lithographic apparatus comprising;
an illuminator configured to perforin a plurality of exposures on a substrate, each exposure being carried out on an exposure area on the substrate and forming one or more markers within the exposure area, wherein neighboring exposure areas share at least one overlapping region;
a measurement device configured to measure the overlay between the markers formed from the neighboring exposures in the overlapping regions and to measure the positions of the markers across the substrate; and
a computing device configured to combine the differences in the measured positions of the markers between exposures in the overlapping regions with the measured positions of the other markers to obtain absolute values of system parameter errors and to derive calibration correction factors from the obtained absolute values of system parameter errors.