Delta TCC for fast sensitivity model computation

1. A method for determining a difference between a reference image and a further image of a pattern, the method comprising:

determining a reference imaging function;

determining parameters of a difference function representative of a difference between the reference imaging function and a further imaging function;

calculating a difference between the reference image and the further image of the pattern based on the difference function and the determined parameters; and

determining the further imaging function based upon the reference imaging function and the difference function,

wherein the further imaging function is determined by concatenating the reference imaging function and the difference function.

2. The method according to claim 1 , wherein calculating the difference in the reference image and the further image of the pattern comprises determining the difference in image intensity of the reference image and the further image of the pattern.

3. The method according to claim 2 , wherein calculating the difference in the reference image and the further image of the pattern further comprises determining the difference between a measure for critical dimensions in the reference image and the further image.

4. The method according to claim 1 , comprising representing a plurality of the terms of the difference function in eigenfunctions and corresponding eigenvalues, and determining a simplified difference function by disregarding certain of the eigenfunctions and eigenvalues based upon the eigenvalues.

5. The method according to claim 4 , wherein the simplified difference function comprises less than 300 eigenvalues or less than 100 eigenvalues.

6. The method according to claim 1 , wherein the reference image models an image of the pattern formed by a reference optical system under reference conditions, and the further image models an image of the pattern formed by a further optical system, or formed by the reference optical system under further conditions, or formed by a modified reference optical system.

7. The method according to claim 6 , comprising decreasing the difference between the reference image and the further image by changing the conditions of the further optical system, the reference optical system or the modified reference optical system.

8. The method according to claim 7 , wherein the reference optical system is a projection system of a lithographic exposure apparatus.

9. A non-transitory computer readable medium having machine executable instructions stored thereon, the instructions to be executed by a machine to perform a method for determining a change in an image of a mask pattern as in any one of the preceding claims 1 to 8 .

10. A method according to claim 1 , wherein determining the reference imaging function comprises:

determining a first matrix of transmission cross-coefficients (TCCs) associated with a base state of a reference optical system;

diagonalizing the first matrix of TCCs to obtain a diagonalized set of TCC kernels, each having eigenfunctions and eigenvalues; and

truncating the diagonalized set of TCC kernels based on the eigenvalues.

11. A method for determining a difference between a reference image and a further image of a pattern, the method comprising:

determining a reference imaging function associated with a base set of optical settings of a first lithographic apparatus;

determining a difference function associated with a difference between a perturbed state of optical settings of a second lithographic apparatus and the base set of optical settings of the first lithographic apparatus;

determining a further imaging function based upon the reference imaging function and the difference function; and

calculating the difference between the reference image and the further image using the reference imaging function and the further imaging function,

wherein the further imaging function is determined by concatenating the reference imaging function and the difference function.

12. The method according to claim 11 , wherein the first lithographic apparatus is a reference lithographic apparatus, and the second lithographic apparatus is a further lithographic apparatus.

13. The method according to claim 11 , wherein the first lithographic apparatus and the second lithographic apparatus are the same reference lithographic apparatus.

14. A method according to claim 11 , wherein determining the reference imaging function comprises:

determining a first matrix of transmission cross-coefficients (TCCs) associated with the base state of the first lithographic apparatus;

diagonalizing the first matrix of TCCs to obtain a diagonalized set of TCC kernels, each having eigenfunctions and corresponding eigenvalues; and

truncating the diagonalized set of TCC kernels based on the eigenvalues.

15. A method according to claim 14 , wherein determining the difference function comprises:

determining a second matrix of transmission cross-coefficients (TCCs) associated with the perturbed state of second lithographic apparatus;

subtracting the first matrix of TCCs from the second matrix of TCCs to obtain a third matrix of TCCs;

diagonalizing the third matrix of TCCs to obtain a diagonalized set of delta TCC kernels, each having eigenfunctions and corresponding eigenvalues; and

truncating the diagonalized set of delta TCC kernels based on the eigenvalues.

16. A method according to claim 15 , wherein determining the further imaging function comprises concatenating together the diagonalized set of TCC kernels and the diagonalized set of delta TCC kernels.

17. A method according to claim 16 , wherein calculating the difference comprises:

calculating a reference aerial image using the reference imaging function;

calculating a further aerial image using the further imaging function; and

subtracting the reference aerial image from the further aerial image.

18. A method according to claim 16 , wherein calculating the difference comprises:

calculating a reference critical dimension using the reference imaging function;

calculating a further critical dimension using the further imaging function; and

subtracting the reference critical dimension from the further critical dimension.