Reflective Mask Blank Absorber Shadowing Reduction
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Solution Overview
Problem
In EUV lithography, the shadowing effect caused by obliquely incident EUV light on absorber patterns in reflective masks leads to inaccuracies in pattern transfer, particularly due to the difficulty in forming fine patterns with existing materials like Ta and Cr-based films, which suffer from thick resist films and etching challenges.
Innovation Solution
A reflective mask blank is designed with a multilayer reflective film, an absorber film comprising a buffer layer made of Ta or Si and an absorption layer of Cr, and an etching mask film, where the buffer layer has a thickness of 0.5 nm to 25 nm and the absorption layer has a thickness of 25 nm or more and less than 60 nm, optimized to reduce shadowing effects and achieve high accuracy.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a thick absorber pattern is used in binary-type reflection mask, then EUV light absorption is sufficient, but pattern dimension accuracy deteriorates due to shadowing effect
Solution Approach 1:
The absorber film is segmented into two distinct layers: a lower absorption layer (Ta or TaN) with thickness of 5-20 nm that provides foundational absorption, and an upper absorption layer (Cr or CrN) with thickness of 20-50 nm that enhances absorption while maintaining profile accuracy. This segmentation allows each layer to contribute differently to the overall function, reducing shadowing effect while ensuring sufficient EUV light absorption.
Solution Approach 2:
The invention uses composite material structure combining Ta-based material (lower absorption layer) and Cr-based material (upper absorption layer). Ta provides good adhesion to the underlying multilayer film and moderate absorption, while Cr adds enhanced absorption capability with better etch selectivity. The composite structure achieves optimal balance between absorption performance and pattern accuracy by leveraging the complementary properties of both materials.
2Manufacturing precision
If absorber film thickness is reduced to minimize shadowing effect, then pattern accuracy improves, but EUV light absorption becomes insufficient
Solution Approach 1:
Rather than using a single thin layer, the absorption function is segmented across two layers with different materials and thicknesses. The lower Ta-based layer (5-20 nm) provides a foundation that adheres well to the multilayer film, while the upper Cr-based layer (20-50 nm) contributes stronger absorption. This segmentation enables achieving sufficient total absorption (reflectance ≤ 2%) while keeping the overall profile thin enough to minimize shadowing effects.
Solution Approach 2:
The invention changes the material composition parameter of the absorber film, transitioning from single-material films to dual-material composite films. By selecting Ta (extinction coefficient k ≈ 0.045) for the lower layer and Cr (extinction coefficient k ≈ 0.085) for the upper layer, the design optimizes the balance between absorption efficiency and shadowing reduction. The specific thickness parameters (lower layer: 5-20 nm, upper layer: 20-50 nm) are carefully chosen to achieve the target reflectance while maintaining pattern accuracy.
3Reliability
If Cr-based absorber film is used to improve absorption, then EUV light absorption increases, but etching difficulty and resist film thickness increase
Solution Approach 1:
The absorber film is divided into two layers with Cr-based material positioned in the upper layer (20-50 nm thickness). This segmentation allows the Cr layer to provide enhanced EUV absorption while the Ta-based lower layer serves as an etching-friendly foundation. The layered structure enables selective etching processes where the Cr layer can be precisely removed or modified without compromising the underlying Ta layer, thus improving overall manufacturability.
Solution Approach 2:
Different regions of the absorber film have different material compositions optimized for different functions: the lower layer (Ta-based) provides good adhesion and moderate absorption with favorable etching characteristics, while the upper layer (Cr-based) provides enhanced absorption. This local quality differentiation allows each part of the film to be optimized for its specific role, balancing absorption performance with etching ease.
4Ease of manufacture
If Ta-based absorber film is used for easy etching, then etching ease improves, but EUV light absorption is insufficient
Solution Approach 1:
The absorber film is segmented into a lower Ta-based layer (5-20 nm) that provides good etching characteristics and adhesion, and an upper Cr-based layer (20-50 nm) that contributes stronger EUV absorption. This segmentation allows the Ta layer to facilitate easy etching while the Cr layer compensates for any insufficient absorption, achieving both ease of manufacture and sufficient absorption performance.
Solution Approach 2:
The invention employs composite materials combining Ta-based material (lower layer) and Cr-based material (upper layer) to achieve synergistic effects. Ta provides excellent etching ease and good adhesion to the multilayer film, while Cr adds enhanced absorption capability. The composite structure achieves reflectance ≤ 2% while maintaining manufacturability through the complementary properties of both materials.
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
The solution enables the formation of fine and highly accurate absorber patterns with reduced shadowing effects, allowing for precise pattern transfer and improved semiconductor device manufacturing, with the absorber film having a reflectance of 2% or less in EUV light.
Implementation Method 1
The reflective mask has a multilayer reflective film for reflecting exposure light
Implementation Method 2
The transfer pattern of the phase shift-type reflection mask includes a relatively thin absorber pattern that reduces EUV light by light absorption
Data Source
AI summary
Provided is a reflective mask blank that enables to further reduce the shadowing effect of a reflective mask and form a fine and highly accurate absorber pattern.The reflective mask blank comprising a multilayer reflective film, an absorber film, and an etching mask film disposed on a substrate in this order, wherein the absorber film comprises a buffer layer and an absorption layer provided on the buffer layer, the buffer layer comprises a material comprising tantalum (Ta) or silicon (Si) and a film thickness of the buffer layer is 0.5 nm or more and 25 nm or less, the absorption layer comprises a material comprising chromium (Cr) and an extinction coefficient of the absorption layer with respect to EUV light is higher than the extinction coefficient of the buffer layer with respect to the EUV light, and the etching mask film comprises a material comprising tantalum (Ta) or silicon (Si) and a film thickness of the etching mask film is 0.5 nm or more and 14 nm or less.