Resolution enhancement for high-NA extreme ultraviolet lithography using aperiodic multilayer masks

Multilayer masks for extreme ultraviolet lithography (EUVL) are currently structured as repeated pairs of silicon and molybdenum with a patterned absorber on top. This periodic design has performed well during the 0.33 NA era, and with small changes to layer thicknesses, can be expected to maintain high reflection under high-NA (0.55) conditions. Even so, the 3D nature of the EUV mask leads to optical interactions, which worsen as feature sizes shrink, one example being shadowing. In recent years, a great deal of interest has been placed on material changes to the mask absorber with attPSM, low-n, and index-matched absorbers being proposed. In this work we build off the progress in optimizing absorber material by altering the periodic nature of the mask. An aperiodic mask allows each layer to have unique thickness, allowing careful optimization of reflected intensity, phase, polarization, and spectral bandwidth. We show validation of previously presented aperiodic multilayers using Siemens pxSMO. We find for 18 nm pitch lines in the 8x direction a 26% gain in NILS with a similar 19% gain in the 4x direction compared to a periodic multilayer. While results on a horizontal 18 nm pitch metal layer show 10% or higher ILS gains across a wide range of clips when compared to a periodic multilayer.