Low-n mask progressing insights: focus on isolated features

Abstract

Low-n masks have gained strong interest due to their potential to simultaneously improve dose and imaging contrast for dense clips. We've presented before that for the imaging of isolated features mask bias and assist features are crucial to minimize the focus range through pitch. In this paper we elaborate on aberration sensitivity for different mask absorber types. We observe that even aberration sensitivities can change significantly by changing the mask absorber type for the same use case. We show that even aberration sensitivity and best-focus shifts are coupled and that they can also be solved together by applying mask & target bias and/or assist features. Finally we show how assist-feature position optimization can reduce the impact of odd aberrations on two-bar features.

Interaction between aberrations and mask 3D effects for low-n and Ta-based absorbers in extreme ultraviolet lithography

“…Another effective way of mitigation of best-focus shifts, which could be combined with mask bias/assist features, is by aberration injection. 7 Finally, we will briefly look at how assist features can also help to reduce the impact of odd aberrations on two-bar features.…”

“…We will show how to reduce best-focus shifts and even aberration sensitivity on the mask by applying mask and target bias and assist features. Another effective way of mitigation of best-focus shifts, which could be combined with mask bias/assist features, is by aberration injection 7 . Finally, we will briefly look at how assist features can also help to reduce the impact of odd aberrations on two-bar features.…”