Pre-bias optical proximity correction

1. A method of preparing layout data for optical and process correction (OPC), comprising:

receiving data representing features of a desired layout to be created photolithographically;

designating edges of the layout data for which pre-OPC bias values are to be determined prior to the application of OPC;

for at least one of the edges for which pre-OPC bias values are to be determined, calculating a density of nearby features; and

using a computer, applying the calculated density of nearby features to a calibrated model that correlates the density of nearby features to an OPC correction to determine at least one pre-OPC bias value for the at least one of the edges.

2. The method of claim 1 , followed by the execution of model-based OPC on the layout.

3. The method of claim 1 , wherein the density for the at least one edge is determined by determining an area occupied by the feature of the layout data under a set of concentric functions with varying radii with their centerpoints on the edge.

4. The method of claim 1 , wherein the density for the at least one edge is a density signature comprising the area occupied by the features of the layout data under a number of top-hat functions having different radii and with their centerpoints on the edge.

5. The method of claim 1 , wherein the at least one pre-OPC bias value comprises an initial condition for the application of OPC to the at least one of the edges.

6. A computer-readable storage medium storing computer-executable instructions for causing a computer to perform a method, the method comprising:

receiving data representing features of a desired layout to be created photolithographically;

designating edges of the layout data for which pre-OPC bias values are to be determined prior to the application of OPC;

for at least one of the edges for which pre-OPC bias values are to be determined, calculating a density of nearby features; and

applying the calculated density of nearby features to a calibrated model that correlates the density of nearby features to an OPC correction to determine at least one of the pre-OPC bias value for the at least one of the edges.

7. The computer-readable storage medium of claim 6 , wherein the method further comprises executing model-based OPC on the layout.

8. The computer-readable storage medium of claim 6 , wherein the density for the at least one edge is determined by determining an area occupied by the features of the layout data under a set of concentric functions with varying radii with their centerpoints on the edge.

9. The computer-readable storage medium of claim 6 , wherein the density for the at least one edge is a density signature comprising the area occupied by the features of the layout data under a number of top-hat functions having different radii and with their centerpoints on the edge.

10. The computer-readable storage medium of claim 6 , wherein the at least one pre-OPC bias value comprises an initial condition for the application of OPC to the at least one of the edges.

11. A method comprising:

receiving layout data representing a desired layout design or portion thereof, the layout data defining one or more polygons;

fragmenting the one or more polygons into one or more edge fragments;

determining at least one density signature of the one or more edge fragments; and

using a computer, determining at least one pre-OPC bias value based at least on the at least one density signature.

12. The method of claim 11 , wherein the at least one pre-OPC bias value is further based on a set of calibrated coefficients, the calibrated coefficients being determined from density signatures of edge fragments in a test layout and a set of known OPC corrections to the edge fragments in the test layout.

13. The method of claim 11 , wherein the determining the at least one density signature comprises convolving at least one of the one or more polygons with a set of radially symmetric functions.

14. The method of claim 13 , wherein at least one of the radially symmetric functions is a top-hat kernel.

15. The method of claim 13 , wherein the radially symmetric functions are centered over at least one simulation site associated with the one or more edge fragments.

16. The method of claim 11 , wherein the determining the at least one pre-OPC bias value comprises multiplying at least one density signature value with at least one calibrated coefficient.

17. The method of claim 11 , further comprising determining one or more OPC corrections for the one or more edge fragments using the at least one pre-OPC bias value.

18. The method of claim 17 , further comprising modifying the placement of at least one edge fragment of the one or more polygons based on the one or more OPC corrections.

19. The method of claim 18 , further comprising creating a photomask based at least on part on the modified placement of the at least one of the one or more edge fragments.

20. The method of claim 19 , further comprising fabricating a chip using the photomask.

21. A computer-readable storage medium storing computer-executable instructions for causing a computer to perform a method comprising:

receiving layout data representing a desired layout design or portion thereof, the layout data defining one or more polygons;

fragmenting the one or more polygons into one or more edge fragments;

determining at least one density signature of the one or more edge fragments; and

with a computer, determining at least one pre-OPC bias value based at least on the at least one density signature.

22. The computer-readable storage medium of claim 21 , wherein the determining the at least one pre-OPC bias value is further based on a set of calibrated coefficients, the calibrated coefficients being determined from density signatures of edge fragments in a test layout and a set of known OPC corrections to the edge fragments in the test layout.

23. The computer-readable storage medium of claim 21 , wherein the determining the at least one density signature comprises convolving the one or more polygons with a set of radially symmetric functions.

24. A system, comprising:

means for receiving layout data representing a desired layout design or portion thereof, the layout data defining one or more polygons;

means for fragmenting the one or more polygons into one or more edge fragments;

means for determining, at least one density signature of the one or more edge fragments; and

means for determining at least one pre-OPC bias value based at least on the at least one density signature and a set of calibrated coefficients.