EUV Mask Sub-Resolution Gratings for High-NA Focus Control
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Solution Overview
Problem
EUV lithography faces challenges with limited depth of focus and variations in best focus across different pitches due to three-dimensional mask topology, exacerbated by high and hyper numerical apertures, and existing solutions like sub-resolution assist features (SRAFs) introduce defects and complexity.
Innovation Solution
A mask design incorporating sub-resolution gratings that are not printable at high or hyper numerical apertures, optimized for best focus, highest contrast, and depth of focus through pitch, combined with a computer-aided method to simulate and select optimal mask layouts.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If sub-resolution assist features (SRAFs) are placed to improve focus variation and increase normalized image log slope (NILS), then image contrast is improved, but the placement becomes complex requiring specialized tools and SRAFs can print unintentionally leading to defects
Solution Approach 1:
The patent extracts the beneficial optical function of SRAFs (improving NILS and focus variation) while removing their harmful printing effect by designing them as sub-resolution features that fall below the process resolution limit. This allows the assist features to provide optical support without being printed on the wafer, thus avoiding defects while maintaining the improved image contrast.
Solution Approach 2:
The patent changes the dimensional parameters of the assist features to be below the resolution threshold of the lithography process. By controlling the size and pitch of these features to be sub-resolution, they provide optical benefits through diffraction and interference effects without being printed, thus resolving the contradiction between improving NILS and avoiding printing defects.
2Manufacturing precision
If low refractive index (low-n) absorber materials are used to increase reflectivity and diffraction efficiency, then image sharpness is improved, but best focus drift occurs for larger pitches worsening the overall focus range
Solution Approach 1:
The patent applies different optical properties to different regions of the mask by using low-n absorber materials specifically in areas where improved sharpness is needed, while the sub-resolution gratings are strategically placed to counteract the focus drift effect. This localized application allows optimization of image sharpness in critical areas while maintaining focus consistency across different pitch regions.
3Manufacturing precision
If numerical aperture (NA) is increased to achieve finer resolution, then resolution is improved, but depth of focus (DoF) becomes extremely limited
Solution Approach 1:
The patent addresses the DoF limitation by introducing sub-resolution gratings that operate in the optical domain rather than relying solely on geometric focus. These gratings create diffracted orders that can be captured even when the direct focus range is limited, effectively extending the useful depth of focus by utilizing a different physical dimension (diffraction angles) to recover signal from out-of-focus regions.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
Enhances image contrast and depth of focus uniformly across various pitches, stabilizes 0th order phase, and reduces complexity, while being compatible with existing EUV lithography tools and materials.
Implementation Method 1
one or more sub-resolution gratings having sub-resolution grating lines, the sub-resolution grating lines extending substantially in a second direction of the mask lay-out
Implementation Method 2
Due to the unique optical properties at EUV wavelengths, the structures on the mask must be multiples of the wavelength thick, resulting in a three-dimensional mask topology
Data Source
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AI summary
The present invention relates to a mask 100 for extreme ultraviolet (EUV) lithography processes in a high or hyper numerical aperture range. The mask 100 includes a mask pattern derived from the superposition of two sub-lay-outs 120, 130. The first sub-lay-out 120 comprises a plurality of mask features intended for printing, with at least one feature oriented along a first direction of the mask lay-out. The second sub-lay-out 130 includes one or more sub-resolution gratings 140 with grating lines that extend substantially in a second direction of the mask lay-out, covering nearly the full width in this second direction. These sub-resolution gratings 140 are designed to be non-printable within the high or hyper numerical aperture range. The invention aims to improve the lithographic performance by optimizing the mask design for specific imaging conditions, such as best focus, highest contrast, and depth of focus through pitch considerations.