SMO photomask inspection in the lithographic plane

Source Mask Optimization (SMO) describes the co-optimization of the illumination source and mask pattern in the frequency domain. While some restrictions for manufacturable sources and masks are included in the process, the resulting photomasks do not resemble the initial designs. Some common features of SMO masks are that the line edges are heavily fragmented, the minimum design features are small and there is no one-to-one correspondence between design and mask features. When it is not possible to link a single mask feature directly to its resist counterpart, traditional concepts of mask defects no longer apply and photomask inspection emerges as a significant challenge. Aerial Plane Inspection (API) is a lithographic inspection mode that moves the detection of defects to the lithographic plane. They can be deployed to study the lithographic impact of SMO mask defects. This paper briefly reviews SMO and the lithography inspection technologies and explores their applicability to 22nm designs by presenting SMO mask inspection results. These results are compared to simulated wafer print expectations.