Calibration and verification structures for use in optical proximity correction

1. A method of training an Optical Proximity Correction (OPC) model comprising the steps of:

symmetrizing a complex design to be a test pattern having orthogonal symmetry;

providing a plurality of complex test patterns representative of real chip designs;

printing the test patterns on a wafer;

measuring the test patterns;

comparing real chip original design with the measurements;

using the results of the comparison to calibrate the OPC model;

clipping the test patterns from real designs, thereby forming one or more clips; and

symmetrizing the clips;

wherein the step of symmetrizing comprises the steps of:

finding the center of the clip; and

if the clip is not symmetric, altering it to make it symmetric.

2. The method of claim 1 , further comprising the step of:

after symmetrizing the test pattern, biasing it up or down and adding biased versions of it to a library of symmetrized test patterns.

3. The method of claim 1 , further comprising the step of:

after symmetrizing the test pattern, adding expanded and shrunk versions of it to a library of symmetrized test patterns.

4. The method of claim 1 , further comprising the step of:

selecting the real chip designs as SRAMs.

5. A method of training an Optical Proximity Correction (OPC) model comprising the step of:

symmetrizing a complex design to be a test pattern having orthogonal symmetry, wherein the step of symmetrizing comprises the steps of:

establishing an axis of symmetry passing through the design, thereby dividing the design into two portions;

deleting one of the two portions; and

mirror-imaging the other of the two portions about the axis of symmetry.

6. The method of claim 5 , further comprising the steps of:

establishing a second axis of symmetry, orthogonal to the first axis of symmetry and passing through the design, thereby dividing the design into second two portions;

deleting one of the second two portions;

minor-imaging the other of the second two portions about the axis of symmetry.

7. The method of claim 5 , the step of:

passing the axis of symmetry through elements of the design.

8. The method of claim 5 , further comprising the step of:

rotating the test pattern.

9. The method of claim 5 , further comprising the step of:

scaling the test pattern.