Sub-Resolution Gratings for High-NA EUV Depth-of-Focus Uniformity
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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 numerical aperture, leading to defects and increased stochastic failure rates.
Innovation Solution
Incorporation of sub-resolution gratings with specific pitches and sizes that are outside the resolution limit of EUV lithography, optimizing focus and contrast by extending in a direction perpendicular to the main features, and using anamorphic optics to demagnify patterns differently in two directions.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If high numerical aperture is used to achieve finer resolution, then resolution is improved, but depth of focus becomes extremely limited
Solution Approach 1:
The patent introduces sub-resolution gratings that extend in the vertical dimension (perpendicular to the mask surface) to address focus variations. These gratings have features that are sub-resolution in the horizontal plane but create optical effects that extend focus control into the vertical dimension, thereby improving depth of focus without sacrificing resolution.
Solution Approach 2:
The patent modifies the optical parameters of the mask by introducing gratings with specific pitch and depth values. By carefully selecting the grating pitch (smaller than the wavelength) and depth, the system changes the diffraction characteristics to compensate for focus variations across different pitch regions, thereby extending the depth of focus while maintaining high resolution.
2Adaptability or versatility
If three-dimensional mask topology is used to encode patterns, then pattern encoding capability is improved, but variations in optimal focus position occur across different pattern pitches
Solution Approach 1:
The patent applies different grating configurations to different regions of the mask based on their specific pitch requirements. Each region with different pattern pitch receives customized sub-resolution grating parameters (pitch, depth, orientation) that are optimized for that specific pitch, thereby achieving local focus optimization across the entire mask while maintaining the ability to encode diverse patterns.
Solution Approach 2:
The mask is segmented into multiple sub-layouts, each containing sub-resolution gratings optimized for specific pitch ranges. This segmentation allows different pitch regions to have their focus characteristics independently optimized, reducing overall focus variation while preserving the comprehensive pattern encoding capability of the three-dimensional mask topology.
3Manufacturing precision
If sub-resolution assist features are used to improve focus variation, then normalized image log slope is improved, but placement complexity increases and unintentional printing occurs
Solution Approach 1:
The patent designs sub-resolution gratings that serve multiple functions simultaneously: they improve normalized image log slope for focus variation, provide depth of focus extension, and act as focus control elements across different pitch regions. By making these features multi-functional, the patent reduces the need for separate specialized elements, thereby simplifying placement while maintaining high imaging quality.
Solution Approach 2:
The patent carefully selects specific parameter ranges for the sub-resolution gratings (pitch slightly smaller than wavelength, controlled depth) to ensure they remain sub-resolution and do not print unintentionally. By optimizing these parameters, the system achieves the desired imaging improvements without triggering unwanted printing, thereby reducing placement complexity and defect rates.
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 uniformity across various pitches, reducing defects and improving lithography process quality, compatible with high and hyper numerical aperture ranges.
Implementation Method 1
the introduction of Extreme Ultraviolet (EUV) lithography, with its 13.5 nm wavelength, marked a significant leap forward in this respect
Implementation Method 2
using anamorphic optics to demagnify patterns differently in two directions
Data Source
AI summary
Example embodiments relate to sub-resolution gratings in extreme ultraviolet (EUV) imaging. One example embodiment includes a mask for EUV lithography processes in a high or hyper numerical aperture range. The mask includes a mask pattern according to a mask layout. The mask layout is a superposition of a first mask sub-layout that includes a plurality of mask features to be printed and a second mask sub-layout that includes one or more sub-resolution gratings having sub-resolution grating lines. The plurality of mask features to be printed includes at least one mask feature oriented along a first direction of the mask layout. The sub-resolution grating lines extend substantially in a second direction of the mask layout and run over substantially a full width in the second direction of the mask layout. The one or more sub-resolution gratings are not printable for the high or hyper numerical aperture range.