Graded Interface in Bragg Reflector for EUV Lithography
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Solution Overview
Problem
Bragg reflectors used in extreme ultraviolet lithography suffer from reduced reflectivity due to the formation of distinct interfacial layers with low reflectivity and higher roughness, which scatter incident light and decrease overall reflectance.
Innovation Solution
The introduction of graded interfacial layers with a density or composition gradient between alternating reflective layers in the multilayer stack of the Bragg reflector, which reduces the formation of low-reflectivity interfaces and smooths out interfacial roughness, enhancing reflectivity.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If distinct interfacial layers form between alternating layers during multilayer deposition, then the multilayer structure is clearly defined, but reflectance decreases due to low reflectivity interfaces and increased roughness
Solution Approach 1:
The patent applies local quality by creating interfacial layers with non-uniform density distributions within each layer. The density varies continuously or in steps across the interface, with higher density regions closer to one material and lower density regions closer to the other, optimizing local optical properties to maximize reflectance while maintaining clear interface definition
Solution Approach 2:
The patent changes the density parameter within interfacial layers to resolve the contradiction. By controlling density gradients and distributions in the interfacial layers through deposition conditions (such as sputtering power, temperature, or gas flow ratios), the patent achieves both well-defined interfaces and high reflectance by preventing excessive roughness and minimizing scattering
2Ease of manufacture
If conventional multilayer deposition is used, then manufacturing is simpler, but reflectance is reduced due to interfacial roughness scattering light
Solution Approach 1:
The patent modifies deposition parameters such as sputtering power, substrate temperature, or gas flow ratios during the deposition process to create interfacial layers with optimized density distributions. These parameter changes are implemented within existing deposition equipment and processes, maintaining ease of manufacture while significantly improving reflectance by reducing interfacial roughness
Solution Approach 2:
The patent applies preliminary action by controlling deposition conditions during the formation of interfacial layers to pre-establish optimal density distributions before the final multilayer structure is complete. This preliminary control of interface properties during deposition ensures high reflectance is achieved throughout the entire multilayer stack
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 use of graded interfacial layers increases the reflectance of the multilayer stack by greater than or equal to 2% for EUV mask blanks, improving the efficiency of extreme ultraviolet light reflection.
Implementation Method 1
Bragg reflectors are typically made of multilayers of alternating thin film materials of different refractive index, wherein high reflectance is one of the key attributes
Implementation Method 2
the distinct interfacial layers have higher roughness than the alternating layers, resulting in interfacial roughness that scatters incident light in random directions
Data Source
AI summary
A multilayer stack in the form of a Bragg reflector comprising a graded interfacial layer and a method of manufacturing are disclosed. The graded interfacial layer eliminates the formation of low-reflectivity interfaces in a multilayer stack and reduces roughness of interfaces in a multilayer stack.