Aperiodic multilayer masks for M3D mitigation in high- and hyper-NA extreme ultraviolet lithography

As extreme ultraviolet lithography tools with higher image numerical apertures (NA_i) are introduced, the range of angles at the multilayer mask stack is also increased. Lithography systems are designed to fulfill the “abbe sine rule,” where NA_m is related to NA_i by the reduction factor. As a result, increases in NA_i will increase NA_m. High-NA and hyper-NA systems will be implemented with anamorphic optics, 4× in “X,” and 8× in “Y” to reduce the necessary angles. Even so, hyper-NA masks may see illumination angles as high as 13.4°, up from 10.8° for 0.33 NA. This represents a challenge for maintaining through-angle mask reflection using the current periodic mask multilayer structures. In addition to the reflectance amplitude, the phase of reflected light, which plays an important role in imaging, is also strongly influenced by increasing angles. The propagation of light through each bilayer in the stack imparts a phase shift based on the incidence angle. This is then accumulated over many layers, inducing phase effects, which are unique to each illumination point. This will become especially true for high-NA and hyper-NA mask applications. While adjustments to the multilayer period are sufficient to achieve acceptable reflectance, periodic multilayers may suffer in normalized image log slope (NILS) and image placement error (IPE) metrics as a result of strong oblique multilayer M3D phase effects. In this work, we present a first principles methodology for the systematic reduction of oblique multilayer M3D effects through the use of aperiodic multilayer design. We find that when used for low-k1, hyper-NA patterning NILS is improved by 40+%, while IPE through pitch is similarly improved. Under high-NA conditions, a 5%–25% NILS gain is found alongside the enhanced depth of focus.