Experimental imaging comparison at 0.55 NA of a low-reflectivity low-n mask to a standard Ta-based EUV mask

Abstract

The successful introduction of extreme ultraviolet (EUV) lithography to high-volume manufacturing has significantly increased interest in pushing this technology to its limits. Standard Ta-based EUV masks suffer from mask 3D (M3D) effects that limit contrast. This contrast loss is primarily caused by aerial image fading, driven by phase shifts between the aerial images of different point sources in the illumination pupil. These phase shifts are influenced by the thickness and refractive index of the absorber material, leading to contrast loss, feature-dependent Best Focus shifts, and asymmetric process windows. Tuning the mask absorber material offers a promising approach to mitigate performance degradation caused by M3D effects. While previous studies have evaluated the benefits of low-reflectivity low-n masks at 0.33 NA for logic applications, this work extends the investigation to high-NA. We manufacture the 6% reflective low-n mask and evaluate its high-NA printing performance to compare it to a conventional Ta-based mask. A bright field mask tonality is selected to align with existing high-NA integration flows. We present an experimental comparison of EUV imaging performance using an EXE:5000 EUV exposure tool with a 0.55 numerical aperture (NA). The study targets critical patterning building blocks for logic metal (20 nm pitch) and DRAM (28 nm pitch) applications. We compare a 6% reflective low-n mask stack to the Ta-based reference. Key metrics include Exposure Latitude, LWR, Best Focus shift through pitch, and Overlapping Process Window for line/spaces features. Additionally, we evaluate Mask Error Enhancement Factor, 2-bar asymmetry through focus, and contact hole array process window. By experimentally demonstrating the patterning performance and advantages of 6% reflective low-n absorber masks for logic metal and DRAM applications, this study contributes to the broader goal of developing and experimentally verifying an EUV mask roadmap for single print Logic and DRAM patterning in low-, high-, and hyper-NA regimes.