Full-field source and mask optimization using gradient surgery strategy

The aberrations of the lithography objective are unevenly distributed at different field points. Full-field source and mask optimization (FFSMO) can reduce the impact of uneven aberration on projection lithography imaging. However, gradient conflicts between field points would prevent FFSMO merit function from converging to an optimal solution. In this paper, the gradient surgery (GS) theory is applied to solve the gradient conflict problem in FFSMO. To mitigate the gradient conflict, the proposed GS-FFSMO method projects the gradient of each field point onto the gradient normal plane of any other field point that has a conflicting gradient. Then, the source and mask patterns based on the corrected gradient information are updated. Compared with traditional FFSMO, the proposed method can effectively improve the lithography imaging fidelity and uniformity at full exposure field.