Technique for analyzing a reflective photo-mask

1. A computer-implemented method for calculating reflected light from a multilayer stack in a reflective photo-mask, comprising:

determining, using a computer, contributions from multiple discrete cells in a model of the multilayer stack in the reflective photo-mask to the reflected light based on angles of incidence of light in a light pattern to the multilayer stack, a polarization of the light in the light pattern, and a varying intensity of the light in the light pattern through the multilayer stack, wherein the multiple discrete cells are at horizontal and vertical positions in the multilayer stack, and wherein the multilayer stack includes at least one defect;

adjusting phase values of the contributions to the reflected light from the multiple discrete cells, thereby specifying optical path differences between the multiple discrete cells in the multilayer stack that are associated with the defect;

combining the contributions to the reflected light from multiple discrete cells to determine the reflected light from the multilayer stack; and

selectively shifting k-space representations of the contributions to the reflected light from the multiple discrete cells based on the angles of incidence, thereby accounting for phase variations associated with deviations from normal incidence.

2. The method of claim 1 , wherein the horizontal and vertical positions are included in horizontal planes in the multilayer stack that are separated by a spacing.

3. The method of claim 1 , wherein determining the reflected light from a given discrete cell in the multiple discrete cells uses a closed-form expression for the reflected light.

4. The method of claim 3 , wherein the closed-form expression treats the given discrete cell as an infinite two-dimensional sheet in the multilayer stack.

5. The method of claim 1 , wherein the defect is associated with vertical displacements or a change in a thickness of one or more layers in a region in the multilayer stack.

6. The method of claim 1 , wherein, for a given discrete cell, the varying intensity of the light pattern through the multilayer stack involves calculating cumulative reflection and absorption by intervening discrete cells between the given discrete cell and a top surface of the multilayer stack.

7. The method of claim 1 , wherein, prior to determining the contributions to the reflected light from the multiple discrete cells, the method further comprises calculating absorption of incident light to the reflective photo-mask by an absorption layer in the model, which is deposited on the multilayer stack in the reflective photo-mask, thereby determining the light pattern incident to the multilayer stack, wherein the absorption layer includes a mask pattern of the reflective photo-mask.

8. The method of claim 7 , wherein the incident light is represented by a plane wave.

9. The method of claim 7 , wherein the light pattern includes a near-field diffraction pattern associated with the mask pattern.

10. The method of claim 7 , wherein, after combining the contributions to the reflected light from multiple discrete cells, the method further comprises calculating absorption of the reflected light from the reflective photo-mask by the absorption layer, thereby calculating reflected light from the reflective photo-mask.

11. The method of claim 10 , wherein the method further comprises calculating an aerial image at an image plane of an optical path in a photolithographic process using a forward optical calculation based on the reflected light from the reflective photo-mask, information about the optical path in the photolithographic process and conditions associated with the photolithographic process.

12. The method of claim 11 , wherein the method further comprises calculating an estimated resist pattern based on the aerial image and a model of a photoresist in the photolithographic process.

13. The method of claim 12 , wherein the method further comprises:

identifying differences between the estimated resist pattern and a target pattern; and

determining an acceptance condition of the reflective photo-mask based on the identified differences.

14. The method of claim 7 , wherein the method further comprises calculating a modification to the absorption layer based on the defect.

15. A non-transitory computer-program product for use in conjunction with a computer system, the computer-program product comprising a computer-readable storage medium and a computer-program mechanism embedded therein to calculate reflected light from a multilayer stack in a reflective photo-mask, the computer-program mechanism including:

instructions for determining contributions from multiple discrete cells in a model of the multilayer stack in the reflective photo-mask to the reflected light based on angles of incidence of light in a light pattern to the multilayer stack, a polarization of the light in the light pattern, and a varying intensity of the light in the light pattern through the multilayer stack, wherein the multiple discrete cells are at horizontal and vertical positions in the multilayer stack, and wherein the multilayer stack includes at least one defect;

instructions for adjusting phase values of the contributions to the reflected light from the multiple discrete cells, thereby specifying optical path differences between the multiple discrete cells in the multilayer stack that are associated with the defect;

instructions for combining the contributions to the reflected light from multiple discrete cells to determine the reflected light from the multilayer stack; and

instructions for selectively shifting k-space representations of the contributions to the reflected light from the multiple discrete cells based on the angles of incidence, thereby accounting for phase variations associated with deviations from normal incidence.

16. The computer-program product of claim 15 , wherein the defect is associated with vertical displacements or a change in a thickness of one or more layers in a region in the multilayer stack.

17. The computer-program product of claim 15 , wherein, prior to the instructions for determining the contributions to the reflected light from the multiple discrete cells, the computer-program mechanism further includes instructions for calculating absorption of incident light to the reflective photo-mask by an absorption layer in the model, which is deposited on the multilayer stack in the reflective photo-mask, thereby determining the light pattern incident to the multilayer stack, wherein the absorption layer includes a mask pattern of the reflective photo-mask.

18. The computer-program product of claim 17 , wherein, after the instructions for combining the contributions to the reflected light from multiple discrete cells, the computer-program mechanism further includes instructions for calculating absorption of the reflected light from the reflective photo-mask by the absorption layer, thereby calculating reflected light from the reflective photo-mask.

19. The computer-program product of claim 18 , wherein the computer-program mechanism further includes instructions for calculating an aerial image at an image plane of an optical path in a photolithographic process using a forward optical calculation based on the reflected light from the reflective photo-mask, information about the optical path in the photolithographic process and conditions associated with the photolithographic process.

20. The computer-program product of claim 19 , wherein the computer-program mechanism further includes instructions for calculating an estimated resist pattern based on the aerial image and a model of a photoresist in the photolithographic process.

21. The computer-program product of claim 19 , wherein the computer-program mechanism further includes instructions for calculating a modification to the absorption layer based on the defect.

22. A computer system, comprising:

at least one processor;

at least one memory; and

at least one program module, the program module stored in the memory and configured to be executed by the processor to calculate reflected light from a multilayer stack in a reflective photo-mask, the program module including:

instructions for determining contributions from multiple discrete cells in a model of the multilayer stack in the reflective photo-mask to the reflected light based on angles of incidence of light in a light pattern to the multilayer stack, a polarization of the light in the light pattern, and a varying intensity of the light in the light pattern through the multilayer stack, wherein the multiple discrete cells are at horizontal and vertical positions in the multilayer stack, and wherein the multilayer stack includes at least one defect;

instructions for adjusting phase values of the contributions to the reflected light from the multiple discrete cells, thereby specifying optical path differences between the multiple discrete cells in the multilayer stack that are associated with the defect;

instructions for combining the contributions to the reflected light from multiple discrete cells to determine the reflected light from the multilayer stack; and

instructions for selectively shifting k-space representations of the contributions to the reflected light from the multiple discrete cells based on the angles of incidence, thereby accounting for phase variations associated with deviations from normal incidence.