Comparing bright field imaging performance at 0.33 NA of a novel low-reflectivity low-n and 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 near the tools’ resolution limit. 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 near the resolution limit, feature-dependent Best Focus shifts, and asymmetric process windows.

Tuning the absorber material presents a promising route to mitigate M3D-induced contrast loss effects.

In this work, we manufacture and evaluate the printing performance of a bright field EUV mask with a novel low-n absorber with low reflectivity (~3%) and compare it to a Ta-based mask.

The experiments were conducted on an NXE:3400 EUV tool with a 0.33 numerical aperture (NA) and focus on the printing quality of building blocks for logic metal (pitch 28 nm) and DRAM (pitch 36 & 34 nm) applications. We assessed the overlapping process window and pattern placement of trenches through pitch, the Mask Error Enhancement Factor, the pattern asymmetry of 2Bars through focus, and the contrast of hexagonal arrays of contacts.

By experimentally demonstrating the patterning performance and advantages of a low-reflective low-n absorber mask for logic metal and DRAM applications, this work shows the potential of tuning the EUV mask stack to meet future highvolume manufacturing requirements for single print logic and DRAM patterning applications.